Allow mocking nullable return types for 7.1

[shieldo]

Port/ cherry-pick of e393e24 (allowing nullable return types for PHP 7.1) to the 0.9 branch. Also includes support for using the self keyword in a return type.

[coveralls]

Coverage decreased (-0.3%) to 73.523% when pulling 56badcc on shieldo:nullable_return_types_in_09 into 4de7969 on padraic:0.9.

[GrahamCampbell]

I think this was intentionally not put on 0.9, because new features need to go to 1.0?

[davedevelopment]

@GrahamCampbell I just haven't been bothering with 0.9, happy for others to make contributions if they want to stick with the tagged releases.

This will need updating to exclude the ::class syntax to pass on 5.4 etc

[shieldo]

Yeah I'd suggest this isn't a "new feature" per se as it's just keeping equivalent feature parity (i.e. expecting mocking to work no matter the type declarations) with a new version of PHP without dropping anything older.

Will fix up the tests re the ::class syntax.

[coveralls]

Coverage decreased (-0.3%) to 73.523% when pulling 803019b on shieldo:nullable_return_types_in_09 into 4de7969 on padraic:0.9.

[davedevelopment]

Thanks!