fix(response): handle nil or empty string bodies first

The fix in 05a917e1239b27be748b9d61d516ac2069d638e1 didn't work because
the respond_to(:to_str) case is triggered by the empty string, so we
never get to this case. Moving it up the chain seems to fix it.
Merge pull request #13 from travis-ci/hh-handle-empty-string-bodies

fix(response): handle nil or empty string bodies first