fix: case-insensitive 'Content-type' response header detection

[vch-7]

Changed the Content-Type response header detection to be case-insensitive in the getResult function.

Fixes: #314

[jasonkuhrt]

Thanks, test would be great.

[vch-7]

Thanks!

To add more context, the primary use case for this would be when the default fetch config in the GraphQLClient instance is overridden with say, an Axios wrapper, where the Response Header object does not implement a case-insensitive get method by default. I've added this to the issue description as well

Added a test for this use case

[vch-7]

Thanks for the merge @jasonkuhrt! When do you plan the next release?

[jasonkuhrt]

@vch-7 Not sure yet, still need to look into failing release tool stuff.