ResponseException definition and handling
Replace FourOhFourException usage with ResponseException
Case-insensitive request headers dictionary
WSGI headers normalization
Merge branch 'response-exception' into develop

* response-exception:
  Replace FourOhFourException usage with ResponseException
  ResponseException definition and handling
Merge branch 'headers-dict' into develop

* headers-dict:
  WSGI headers normalization
  Case-insensitive request headers dictionary