Upgrade Netty to 4.0.20.Final (#357)
Adding a response interface for the test http client so we don't bleed the internal implementation to the users.

Converted internal tests to use the new HttpResponse
WIP
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Add autoclosable to the TestHttpClient, DefaultTestHttpClient, and BlockingHttpClient

Change TestHttpClient to be based around actions configuring the request.
WIP
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Test Fixes
Changed setRequest to RequestSpec

Worked through ratpackcore testing updates
WIP
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