Fix partial mock override implementation when returning structs; depending on the struct and the architecture
it may be necessary to override with objc_msgForward_stret since that is how the compiler is going to set
up the frame for the call.  When using the wrong one, the compiler will set up the frame differently, and
objc_msgForward will then assume its arguments are in places they are not, leading to crashes very quickly.
Problem mentioned at http://stackoverflow.com/questions/16559191/returning-cgrect-from-mocked-method-crashes .
Merge pull request #41 from carllindberg/FixStructureReturnBug

Fix partial mock override implementation when returning structs