This patch makes sure header overrides in request interception are
functioning as expected.
Drive-by: teach test server to use utf-8 charset header for text files.
This patch:
- implements Response.buffer() and other methods
- splits out relevant tests into a separate test suites
- implements `testServer.enableGzip()` method to optionally gzip
certain routes in tests
- adds tests to make sure `Response.text()` returns expected results
for binary and compressed responses.
