code: function _httpPost( wpreq, data, callback ) {
  	checkMethodSupport( 'post', wpreq );
  	const url = wpreq.toString();
  	data = data || {};
  	let request = _auth( agent.post( url ), wpreq._options, true );
  	request = _setHeaders( request, wpreq._options );
  
  	if ( wpreq._attachment ) {
  		// Data must be form-encoded alongside image attachment
  		request = objectReduce(
  			data,
  			( req, value, key ) => req.field( key, value ),
  			request.attach( 'file', wpreq._attachment, wpreq._attachmentName )
  		);
  	} else {
  		request = request.send( data );
  	}
  
  	return invokeAndPromisify( request, callback, returnBody.bind( null, wpreq ) );
  }
docs: Invoke an HTTP "POST" request against the provided endpoint
  @method post
  @async
  @param {WPRequest} wpreq A WPRequest query object
  @param {Object} data The data for the POST request
  @param {Function} [callback] A callback to invoke with the results of the POST request
  @returns {Promise} A promise to the results of the HTTP request