[PSR-7] Removed references to RFC 2616

- RFC 7230 describes HTTP messages, while RFC 7231 describes specifically
  request and response messages.
Removes trailing spaces from proposal
Merge pull request #346 from weierophinney/hotfix/reason-phrase-reference

[PSR-7] Remove references to RFC 2616
Merge pull request #351 from sagikazarmark/hotfix/trailing-spaces

[PSR-7] Removes trailing spaces from proposal