This is a simple Rack Middleware to set the request content type for specific routes. It checks if a content type is set and does not override the existing one. The options allow you to specify the request methods, path (with a string or a regular expression) and the content type to be set.

api_content_type.rb

module Rack
  class ApiContentType
    def initialize(app, methods = [:post, :patch], path = /^\/api\/v2+/, content_type = 'application/json')
      @app = app
      @methods = (methods.is_a?(Array) ? methods : [methods]).map{ |item| item.to_s.upcase }
      @path = path
      @content_type = content_type
    end

    def call(env)
      req = Rack::Request.new(env)

      if match_path?(req.path) and match_method?(req.request_method)
        env['CONTENT_TYPE'] = @content_type if req.content_type.nil? or req.content_type.empty?
      end

      @app.call(env)
    end

  private

    def match_path?(path)
      @path.is_a?(Regexp) ? @path.match(path.to_s) : @path == path.to_s
    end

    def match_method?(method)
      @methods.empty? || @methods.include?(method)
    end
  end
end

api_content_type_spec.rb

require 'spec_helper'

describe Rack::ApiContentType do
  App = lambda { |env| [200, {'Content-Type' => 'text/plain'}, env['PATH_INFO']] }

  context '/' do
    it 'does not set the content type on default requests' do
      request  = Rack::MockRequest.env_for("/")
      request['CONTENT_TYPE'] = 'text/plain'
      response = Rack::ApiContentType.new(App).call(request)
      request['CONTENT_TYPE'].should == 'text/plain'
    end
  end

  context '/api/v2' do
    it 'does set the content type' do
      request  = Rack::MockRequest.env_for("/api/v2/customers", method: :post)
      response = Rack::ApiContentType.new(App).call(request)
      request['CONTENT_TYPE'].should == 'application/json'
    end

    it 'does not override existing content type' do
      request  = Rack::MockRequest.env_for("/api/v2/customers", method: :post)
      request['CONTENT_TYPE'] = 'application/xml'
      response = Rack::ApiContentType.new(App).call(request)
      request['CONTENT_TYPE'].should == 'application/xml'
    end

    it 'does override empty content type' do
      request  = Rack::MockRequest.env_for("/api/v2/customers", method: :post)
      request['CONTENT_TYPE'] = ''
      response = Rack::ApiContentType.new(App).call(request)
      request['CONTENT_TYPE'].should == 'application/json'
    end
  end

  context 'method' do
    it 'allows to specify the methods' do
      request1  = Rack::MockRequest.env_for("/api/v2/customers", method: :post)
      request2  = Rack::MockRequest.env_for("/api/v2/customers/1", method: :patch)
      response1 = Rack::ApiContentType.new(App, :post).call(request1)
      response2 = Rack::ApiContentType.new(App, :post).call(request2)
      request1['CONTENT_TYPE'].should == 'application/json'
      request2['CONTENT_TYPE'].should_not == 'application/json'
    end

    it 'may be an array' do
      request1  = Rack::MockRequest.env_for("/api/v2/customers", method: :post)
      request2  = Rack::MockRequest.env_for("/api/v2/customers/1", method: :patch)
      response1 = Rack::ApiContentType.new(App, [:post, :patch]).call(request1)
      response2 = Rack::ApiContentType.new(App, [:post, :patch]).call(request2)
      request1['CONTENT_TYPE'].should == 'application/json'
      request2['CONTENT_TYPE'].should == 'application/json'
    end
  end

  context 'path' do
    it 'allows to specify the path' do
      request1  = Rack::MockRequest.env_for("/api/v2/customers", method: :post)
      request2  = Rack::MockRequest.env_for("/api/v2/customers", method: :post)
      response1 = Rack::ApiContentType.new(App, :post, '/api/v2/customers').call(request1)
      response2 = Rack::ApiContentType.new(App, :post, '/api/v2/cases').call(request2)
      request1['CONTENT_TYPE'].should == 'application/json'
      request2['CONTENT_TYPE'].should_not == 'application/json'
    end

    it 'may be a regular expression' do
      request1  = Rack::MockRequest.env_for("/api/v2/customers", method: :post)
      request2  = Rack::MockRequest.env_for("/api/v2/customers", method: :post)
      response1 = Rack::ApiContentType.new(App, :post, '/api/v2/customers').call(request1)
      response2 = Rack::ApiContentType.new(App, :post, /^\/api\/v2+/).call(request2)
      request1['CONTENT_TYPE'].should == 'application/json'
      request2['CONTENT_TYPE'].should == 'application/json'
    end
  end

  context 'content type' do
    it 'allows to specify the content type' do
      request  = Rack::MockRequest.env_for("/api/v2/customers", method: :post)
      response = Rack::ApiContentType.new(App, :post, '/api/v2/customers', 'application/xml').call(request)
      request['CONTENT_TYPE'].should == 'application/xml'
    end
  end
end

spec_helper.rb

ENV['RACK_ENV'] = 'test'

require 'rspec'
require 'rack/test'
require 'simplecov'
SimpleCov.start

$LOAD_PATH.unshift(File.join(File.dirname(__FILE__), '..', 'lib'))
$LOAD_PATH.unshift(File.dirname(__FILE__))
require 'rack/api_content_type'