reduce amount of default logging on a failed request

Thanks for the review @kaisecheng. Your comment made me realize two things:

this code path wasn't actually being tested anywhere in the test suite
the tests I added were covering the part of the code that handled the responses for requests that succeeded and not when client.send raised an exception

I added a new commit that fixes the variable name and also moved the tests to the right place. Now if the variable is not defined, the tests will fail. Because of all of these changes I'm re-requesting a review. thanks!

logstash-plugins / logstash-output-http

reduce amount of default logging on a failed request #122