Fix field merging behavior for fragments on interfaces

[kxfang]

This PR fixes #2379 . The cause appears to be that when merging fields from a fragment spread, the interface equality check needs to be done in both directions, but is only done in one direction in the existing code.

I have:

• Added tests covering the bug / feature (see testing)
• Updated any relevant documentation (see docs)

[coveralls]

Coverage increased (+0.01%) to 75.465% when pulling 4a490b4 on kxfang:kfang/fix_fragment_on_interface_field_merging into 6bb3186 on 99designs:master.

[StevenACoffman]

Thanks very much for this!

[kxfang]

@StevenACoffman I’m glad I could contribute! Any chance of a release in the next day or two?

[davidbyttow]

Thank you @StevenACoffman! (and Kev) ;)