Consider removing WebMock dependency

[pond]

PR https://github.com/LoyaltyNZ/hoodoo/pull/229 introduces a test dependency on WebMock for expediency, but only uses it for this one test alone. It's a heavy dependency to pull in for that one use case; consider instead manually mocking out just the Net::HTTP bits of interest for those tests to slim back down the Gemfile.lock a little.