Update and correct examples to the new response code and correct HTTPResponse code type from Integer to String in the docs and tests

Net::HTTPResponse’s code returns a string, see http://ruby-doc.org/stdlib-2.1.1/libdoc/net/http/rdoc/Net/HTTPResponse.html 

I realise this doesn’t change much as the response code is currently just passed through, but it could be confusing if the tests are used as examples, or something changes where the code is processed.

The examples in the readme put me on the wrong track, this PR fixes that based on my experiences.

I was testing all these build triggering and response codes against Jenkins ver. 1.568

Thanks for this gem!