Groovy HTTPBuilder fails with GitHub

HTTPBuilderGitHubFailure.groovy

@Grab(group='org.codehaus.groovy.modules.http-builder', module='http-builder', version='0.6' )
 
import groovyx.net.http.*
import groovyx.net.http.RESTClient
import static groovyx.net.http.ContentType.URLENC
import static groovyx.net.http.ContentType.*
import static groovyx.net.http.Method.*

// this fails
try {
    def github = new RESTClient( 'https://api.github.com' )
    def resp = github.get( path : '/gists/public' )
    assert resp.status == 200
    assert resp.contentType == JSON.toString()
    assert ( resp.data instanceof net.sf.json.JSON )
}
catch (e) { println e }

// this also fails
try {
    def http = new HTTPBuilder('https://api.github.com')
    http.get( path : '/gists/public',
              contentType : JSON) { response, reader ->
     
      assert resp.status == 200
    }
}
catch (e) { println e }

// this works
new URL('https://api.github.com/gists/public').text