Set HTTPI logger to Savon logger

[krissi]

This PR sets the HTTPI logger to the Savon logger in addition to the log flag.

[tjarratt]

Thanks for the pull request @krissi!

I'm not convinced that the specs for this change are valuable, because it doesn't seem that they directly verify that the HTTPI logger was set correctly. Probably makes more sense to verify that it's the same instance as the Savon logger.

Would you mind making that change before we merge?

[krissi]

I have to admit I did not create any spec for this. The changes were only to let the specs pass again.

I could have fixed the specs to output checking instead of message expectations, but there is a great feature in RSpec 3.1 which allows the following, so I skipped the refactoring of those specs

specify { expect { warn('foo') }.to output("foo\n").to_stderr }

[tjarratt]

Thanks for updating your pull request @krissi! I had hoped to get the tests green on travis-ci before merging this, but it's been a real struggle to accomplish that lately. I feel like this change is pretty simple and not too likely to break anyone else, so I'm more than happy to merge this in.