AFNetworking2.0でContent-Type: plain/textのレスポンスを扱う方法 ref: http://qiita.com/okitsutakatomo/items/21b126fb1e3dae3bcb2f

AFTextResponseSerializer.h

#import "AFURLResponseSerialization.h"

@interface AFTextResponseSerializer : AFHTTPResponseSerializer

@end

AFTextResponseSerializer.m

#import "AFTextResponseSerializer.h"

@implementation AFTextResponseSerializer

+ (instancetype)serializer {
    AFTextResponseSerializer *serializer = [[self alloc] init];

    return serializer;
}

- (instancetype)init {
    self = [super init];
    if (!self) {
        return nil;
    }

    self.acceptableContentTypes = [[NSSet alloc] initWithObjects:@"text/plain", nil];

    return self;
}

#pragma mark - AFURLResponseSerialization

- (id)responseObjectForResponse:(NSHTTPURLResponse *)response
                           data:(NSData *)data
                          error:(NSError *__autoreleasing *)error
{
    if (![self validateResponse:(NSHTTPURLResponse *)response data:data error:error]) {
        if ([(NSError *)(*error) code] == NSURLErrorCannotDecodeContentData) {
            return nil;
        }
    }

    return [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
}

@end

file0.txt

Request failed: unacceptable content-type: text/plain

file1.txt

oauth_callback_confirmed=true&oauth_token=XXXXXXXXXX-XXXXXXXXXX&oauth_token_secret=XXXXXXXXXXX

file4.txt

AFHTTPRequestOperationManager *manager = [AFHTTPRequestOperationManager manager];

// AFTextResponseSerializerをResponseSerializerに設定する。
AFTextResponseSerializer *textSerializer = [AFTextResponseSerializer serializer];
[manager setResponseSerializer:textSerializer];

[manager GET:baseUrl parameters:signedParams success:^(AFHTTPRequestOperation *operation, id responseObject) {
    NSLog(@"Response: %@", responseObject);
} failure:^(AFHTTPRequestOperation *operation, NSError *error) {
    NSLog(@"Error: %@", error);
}];