/**
  * Check whether implementor modified content type or etag header
  * if so throw InvalidOperationException.
  * 
  * @param string $methodName NAme of the method
  * 
  * @return void
  * 
  * @throws InvalidOperationException
  */
 private function _verifyContentTypeOrETagModified($methodName)
 {
     if ($this->_responseContentType !== $this->_service->getHost()->getResponseContentType() || $this->_responseETag !== $this->_service->getHost()->getResponseETag()) {
         throw new InvalidOperationException(Messages::streamProviderWrapperMustNotSetContentTypeAndEtag($methodName));
     }
 }