Update the SessionFetcher Dep.

Update the code to use the helper instead of the mutableRequest api. This
update the code for the mutableRequest change in
google/gtm-session-fetcher#48

Author: thomasvl

Deps/gtm-session-fetcher

@@ -711,7 +711,11 @@ - (GTLUploadFetcherClass *)uploadFetcherWithRequest:(NSURLRequest *)request
 
   NSString *slug = [uploadParams slug];
   if ([slug length] > 0) {
+#if GTL_USE_SESSION_FETCHER
+    [fetcher setRequestValue:slug forHTTPHeaderField:@"Slug"];
+#else
     [[fetcher mutableRequest] setValue:slug forHTTPHeaderField:@"Slug"];
+#endif
   }
   return fetcher;
 }

Source/Objects/GTLService.m

@@ -366,7 +366,7 @@ - (void)testServiceRPCFetch {
     maxUploadProgress = numberOfBytesRead;
   };
 
-  NSString *headerValue = [queryTicket.objectFetcher.mutableRequest valueForHTTPHeaderField:@"X-Feline"];
+  NSString *headerValue = [queryTicket.objectFetcher.request valueForHTTPHeaderField:@"X-Feline"];
   XCTAssertEqualObjects(headerValue, @"Fluffy");
 
   [self service:service waitForTicket:queryTicket];