Updated component to psr/http-message 0.4.0

Updates to use psr/http-message 0.4.0, which contains a number of BC breaks and changes in behavior.

- 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.