Ignore if MIMEType method return nil.

I got NSInvalidArgumentException in my app case.
MIMEType instance method of the NSURLResponse returns nil sometimes.

https://developer.apple.com/library/mac/documentation/cocoa/reference/foundation/classes/nsurlresponse_class/Reference/Reference.html#//apple_ref/occ/instm/NSURLResponse/MIMEType


    MyApp[9066] <Error>: *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[__NSPlaceholderDictionary initWithObjects:forKeys:count:]: attempt to insert nil object from objects[1]'
	*** First throw call stack:
	(
		0   CoreFoundation                      0x0020f1e4 __exceptionPreprocess + 180
		1   libobjc.A.dylib                     0x025688e5 objc_exception_throw + 44
		2   CoreFoundation                      0x001d5376 -[__NSPlaceholderDictionary initWithObjects:forKeys:count:] + 390
		3   CoreFoundation                      0x00202c29 +[NSDictionary dictionaryWithObjects:forKeys:count:] + 73
		4   MyApp                               0x00053b5d -[ChromeURLProtocol connection:didReceiveResponse:] + 221
		5   Foundation                          0x01c90f54 ___NSURLConnectionDidReceiveResponse_block_invoke + 79
		6   Foundation                          0x01c277e1 __65-[NSURLConnectionInternal _withConnectionAndDelegate:onlyActive:]_block_invoke + 62
		7   Foundation                          0x01aaff5e -[NSURLConnectionInternalConnection invokeForDelegate:] + 119
		8   Foundation                          0x01aafec6 -[NSURLConnectionInternal _withConnectionAndDelegate:onlyActive:] + 208
		9   Foundation                          0x01aafdd8 -[NSURLConnectionInternal _withActiveConnectionAndDelegate:] + 76
		10  Foundation                          0x01aaffc9 _NSURLConnectionDidReceiveResponse + 87
		11  CFNetwork                           0x06306eb0 ___ZN27URLConnectionClient_Classic28_delegate_didReceiveResponseEP14_CFURLResponse_block_invoke + 125
		12  CFNetwork                           0x063023de ___ZN27URLConnectionClient_Classic18_withDelegateAsyncEPKcU13block_pointerFvP16_CFURLConnectionPK33CFURLConnectionClientCurrent_VMaxE_block_invoke_2 + 104
		13  CoreFoundation                      0x001b0c69 CFArrayApplyFunction + 57
		14  CFNetwork                           0x0626b441 _ZN19RunloopBlockContext7performEv + 155
		15  CFNetwork                           0x0634d3f4 _ZThn16_N19RunloopBlockContext24multiplexerClientPerformEv + 20
		16  CFNetwork                           0x0626b257 _ZN17MultiplexerSource7performEv + 299
		17  CFNetwork                           0x0626b06c _ZN17MultiplexerSource8_performEPv + 76
		18  CoreFoundation                      0x0019877f __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 15
		19  CoreFoundation                      0x0019810b __CFRunLoopDoSources0 + 235
		20  CoreFoundation                      0x001b51ae __CFRunLoopRun + 910
		21  CoreFoundation                      0x001b49d3 CFRunLoopRunSpecific + 467
		22  CoreFoundation                      0x001b47eb CFRunLoopRunInMode + 123
		23  WebCore                             0x04d49a18 _ZN7WebCoreL15runLoaderThreadEPv + 360
		24  JavaScriptCore                      0x09093b2c _ZN3WTFL16threadEntryPointEPv + 76
		25  JavaScriptCore                      0x09094025 _ZN3WTFL19wtfThreadEntryPointEPv + 21
		26  libsystem_c.dylib                   0x02df45b7 _pthread_start + 344
		27  libsystem_c.dylib                   0x02ddedce thread_start + 34
	)
    backboardd[55407] <Warning>: Application 'UIKitApplication:com.example.myapp[0x854f]' exited abnormally with signal 6: Abort trap: 6