Fix a minor bug in the GraphQL formatter.

[obi1kenobi]

Here's the error I was getting.

File /.../graphql_compiler/query_formatting/graphql_formatting.py", in leave_Directive
    arg = args.pop(arg_names.index(defined_arg))
IndexError: pop index out of range

I wasn't able to come up with a deterministic test case that reproduces the problem, but I know the underlying cause. The problem happens because the args and arg_names lists were expected to be parallel, but popping could happen in any order. The error above is caused when the lists cease to be parallel, e.g. as follows:

********* before first pop, popping at index 0

['op_name: "="', 'value: ["$parameter"]']
['op_name', 'value']

********* before second pop, popping at index 1

['value: ["$parameter"]']
['op_name', 'value']

********* IndexError since there was no index

[obi1kenobi]

@benlongo if you have a moment, since I'm modifying your code, I'd appreciate your feedback as well. Also, thank you for the thorough unit tests you wrote -- they were very helpful in making sure I didn't add more bugs by accident :)

[coveralls]

Coverage increased (+0.007%) to 93.72% when pulling eb6258d on fix_formatting_bug into a36ac09 on master.

[benlongo]

What a curious little bug - the changes look good, I probably should've gone with the set approach to begin with 👍

[obi1kenobi]

Hindsight is 20/20 after all :) Thanks for reviewing so quickly!