fix sigv4 tests to work on httparse 1.5 (#656)

* fix sigv4 tests to work on httparse 1.5

httparse 1.5 has a small behavior change: https://github.com/seanmonstar/httparse/issues/102 which
changes the parsing behavior of headers when the request is not terminated with a second newline.

Although 1.5 may eventually fix this issue, this works around the build failure by inserting an additional
newline into the parser prior to parsing the test requests.

* update changelog

* Proper test fix