Fix required parameter validation for Objc

[geekerzp]

• Added required parameter check for objc client.

  Currently, required parameter validation is missing/broken.
  This PR aims to improve the method/function by ensuring that the required parameter is set.
  Otherwise, an exception will be thrown.

• Tested without issue.

  Test Case '-[PetApiTest testUploadFile]' passed (0.411 seconds).
  Test Suite 'PetApiTest' passed at 2015-05-19 09:04:38 +0000.
   Executed 6 tests, with 0 failures (0 unexpected) in 6.423 (6.426) seconds
  Test Suite 'PetstoreClientTests.xctest' passed at 2015-05-19 09:04:38 +0000.
   Executed 6 tests, with 0 failures (0 unexpected) in 6.423 (6.427) seconds
  Test Suite 'All tests' passed at 2015-05-19 09:04:38 +0000.
   Executed 6 tests, with 0 failures (0 unexpected) in 6.423 (6.429) seconds
  [INFO] ------------------------------------------------------------------------
  [INFO] BUILD SUCCESS
  [INFO] ------------------------------------------------------------------------
  [INFO] Total time: 31.758 s
  [INFO] Finished at: 2015-05-19T17:04:38+08:00
  [INFO] Final Memory: 11M/156M
  [INFO] ------------------------------------------------------------------------