Make it safe to use -mkt_arguments as a generic helper

[0x1eaf]

Currently it is not safe to use -[NSInvocation mkt_arguments] with NSInvocation objects that don't come from OCMockito itself, as it doesn't call -[NSInvocation retainArguments] before adding the argument values to an autoreleased NSArray. Meaning that if the NSInvocation object has a stack block argument, it would be added as is to the NSArray as -[__NSStackBlock__ retain] does nothing. And the program might crash if the autorelease elision optimisation happens to not be performed by the objc runtime on the array, and the array ends up in an autorelease pool outliving the stack block and segfaults when trying to release the block during deallocation when the pool is finally popped.

It is safe to use -mkt_arguments with NSInvocation objects produced by OCMockito itself as -[NSInvocation retainArguments] is automatically called via:

• -[NSInvocation mkt_retainArgumentsWithWeakTarget]
• -[MKTInvocationContainer setInvocationForPotentialStubbing:]
• -[MKTBaseMockObject forwardInvocation:]

It is trivial, safe and cheap to add a call to -retainArguments at the start of -mkt_arguments as it doesn't do anything in case if the arguments have already been retained, which is the case for NSInvocation objects produced by OCMockito itself.

And this makes it safe and convenient to use -mkt_arguments as a generic helper for any NSInvocation objects regardless of their provenance.