Rewrite gcp-function-http-test to support OptionsFilterTest in the TCK

[timyates]

As it currently stands, the OptionFilterTest TCK test has to be excluded from the gcp-function-http-test TCK test suite.

This is because gcp-function-http-test does not provide a method of altering the headers in a response (as required by the OptionsFilter), it presents a Map based copy of the headers in the actual response object (so changes to it are not maintained)

To fix this, we should rewrite gcp-function-http-test to be more like function-aws-api-proxy-test in the AWS module, which I believe will make this test work.