NSURLSession: implement missing body data for data completion handlers

Now using the previously unused "in-memory" body data drain if a task has a completion handler, which requires the full body to be passed on completion.

Also consolidated private NSURLSessionTask methods, some of which were previously implemented twice in separate categories with the same name, leading to possible undefined runtime behavior.

Source/GSHTTPURLProtocol.m

@@ -13,44 +13,8 @@
 #import "Foundation/NSStream.h"
 #import "Foundation/NSURL.h"
 #import "Foundation/NSURLError.h"
-#import "Foundation/NSURLSession.h"
 #import "Foundation/NSValue.h"
 
-
-@interface NSURLSessionTask (Internal)
-
-- (void) setCountOfBytesExpectedToReceive: (int64_t)count;
-
-- (void) setCountOfBytesExpectedToSend: (int64_t)count;
-
-- (dispatch_queue_t) workQueue;
-
-@end
-
-@implementation NSURLSessionTask (Internal)
-
-- (void) setCountOfBytesExpectedToReceive: (int64_t)count
-{
-  _countOfBytesExpectedToReceive = count;
-}
-
-- (void) setCountOfBytesExpectedToSend: (int64_t)count
-{
-  _countOfBytesExpectedToSend = count;
-}
-
-- (GSURLSessionTaskBody*) knownBody
-{
-  return _knownBody;
-}
-
-- (dispatch_queue_t) workQueue
-{
-  return _workQueue;
-}
-
-@end
-
 @interface GSURLCacherHelper : NSObject
 
 + (BOOL) canCacheResponse: (NSCachedURLResponse*)response