fix wrq:response_code/1 to handle custom reason phrase

An attempt to use webmachine tracing crashed due to the wrq:response_code/1
function not handling custom reason phrases introduced in commit
dedc5736. Add a new clause for that function to handle custom reason
phrases.
handle custom reason phrases as iolist() in make_code

Treat custom reason phrases as the iolist() type, not iodata(), in
webmachine_request:make_code/1, as per the documentation.
Merge pull request #147 from basho/sbv-fix-respcode

fix wrq:response_code/1 to handle custom reason phrase, and handle custom reason phrases as iolist() in make_code