Updated component to psr/http-message 0.4.0

- Modified IncomingRequest to only allow arrays for either input or return
  values.
- Modified IncomingRequest to `s/PathParams/Attributes/g`
- Removed setHeaders()/addHeaders() from MessageTrait
- Updated MessageTrait::addHeader() to allow array values
- Removed ability to pass objects to MessageTrait::addHeader()/setHeader()
- Added IncomingRequestFactory::setHeaders() for simplifying setting
  (overwriting) many headers at once from an array.
[0.6.0] Updated changelog
Merge branch 'hotfix/http-message-0.4.0'

Close #13