Refactor test client

[vektah]

There have been a few PRs adding various features to the http client:

• [RFC] Split client.Post into client.NewRequest & client.Do #720 - add a generic Do method
• Hotfix/857 #859 - add headers across whole client
• adding requestHeaders to the websocket client #832 - add headers per request

After #720 we took this client and kept iterating internally - this pr is that code cleaned up and reintegrated with gqlgen.

Notable changes:

• takes a http.Handler instead of a url. This produces better stacktraces on panics and hides the cleanup logic from the user.
• makes http.Request available to Options, this means headers can now easily be added both inside this lib and outside.
• add Options to the client.New method, so options can be applied from across multiple tests easily (eg csrf headers)

I have:

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

[coveralls]

Coverage increased (+0.1%) to 61.949% when pulling 9fff140 on refactor-test-client into 55b2144 on master.