HttpClientConnectionTest: Throw UnsupportedOperationException for unused methods

HttpClientConnectionTest: Throw UnsupportedOperationException for unused methods

The HttpResponseMock class is only used in a single test to assert
on the expected headers. All of its overridden methods are empty, and
this results in warnings in Eclipse:

  'Empty block should be documented'

Rather than adding "// Not used" comments, change all of the methods'
implementations to throw UnsupportedOperationException. This not only
gets rid of the warnings, but also makes it explicit that we don't
intend any of them to actually be called.

Change-Id: I8fe06b155e0809bb3507e4e28b00fcc4f9333b0b
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>