resolver: fix case-insensitive file name collision

[erankor]

for compatibility with Windows/Mac OS X, Go does not allow files with identical case-insensitive names.
this commit changes the key of the 'files' map to use the lower case file name, keeping the original file name as a property on the File object.

Describe your PR and link to any relevant issues.

I have:

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

[coveralls]

coverage: 75.846% (-3.6%) from 79.433% when pulling bfe8569 on erankor:fix-case-insensitive-file-name-collision into 9c5fc30 on 99designs:master.

[erankor]

To add a bit more color on the issue we've encountered -
We have a type Metric in our schema, and we have a query metric for getting a single Metric object. The code of the metric resolver is generated to metric.resolvers.go (=based on the name of metric query).
In addition, the Metric type has a field that accepts parameters. We are using @goField(forceResolver: true) on all parameterized fields, so that it will work as expected if such a field is requested multiple times with different parameters. The code of the field resolver is generated to Metric.resolvers.go (=based on the name of the Metric type)
So - we end up with two files with identical (case-insensitive) names, which Go does not allow.

[StevenACoffman]

Thanks! This is the kind of issue that is easy to overlook, but can cause a lot of frustration, so I appreciate you fixing it. Looking forward to your next PR!

[erankor]

Thanks @StevenACoffman !