Minor improvement to Objc API Client 'accept' and 'content-type' header

[geekerzp]

• Updated Objc API client so that Accept and Content-Type header will use application/json if it's provided in the Swagger spec.
• Tested without issue:

Test Case '-[PetApiTest testUploadFile]' passed (0.774 seconds).
Test Suite 'PetApiTest' passed at 2015-05-23 07:29:02 +0000.
     Executed 6 tests, with 0 failures (0 unexpected) in 14.724 (14.727) seconds
Test Suite 'SWGApiClientTest' started at 2015-05-23 07:29:02 +0000
Test Case '-[SWGApiClientTest testSelectHeaderAccept]' started.
Test Case '-[SWGApiClientTest testSelectHeaderAccept]' passed (0.000 seconds).
Test Case '-[SWGApiClientTest testSelectHeaderContentType]' started.
Test Case '-[SWGApiClientTest testSelectHeaderContentType]' passed (0.000 seconds).
Test Suite 'SWGApiClientTest' passed at 2015-05-23 07:29:02 +0000.
     Executed 2 tests, with 0 failures (0 unexpected) in 0.001 (0.002) seconds
Test Suite 'PetstoreClientTests.xctest' passed at 2015-05-23 07:29:02 +0000.
     Executed 8 tests, with 0 failures (0 unexpected) in 14.725 (14.731) seconds
Test Suite 'All tests' passed at 2015-05-23 07:29:02 +0000.
     Executed 8 tests, with 0 failures (0 unexpected) in 14.725 (14.733) seconds
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 33.549 s
[INFO] Finished at: 2015-05-23T15:29:02+08:00
[INFO] Final Memory: 9M/160M
[INFO] ------------------------------------------------------------------------

Also performed testing by flipping the order of application/xml, application/json in petstore.json and tests are fine.