[PSR-7] Add false as a return value for read()

Per @mtdowling, `StreamableInterface::read()` should allow returning `false`.
Merge pull request #342 from weierophinney/hotfix/psr7-read-return

[PSR-7] Allow returning false from StreamableInterface::read()
[PSR-7] Removed references to RFC 2616

- RFC 7230 describes HTTP messages, while RFC 7231 describes specifically
  request and response messages.