Invoke request transformers after request interceptors

closes #352

lib/core/dispatchRequest.js

@@ -1,5 +1,8 @@
 'use strict';
 
+var utils = require('./../utils');
+var transformData = require('./transformData');
+
 /**
  * Dispatch a request to the server using whichever adapter
  * is supported by the current environment.
@@ -8,6 +11,30 @@
  * @returns {Promise} The Promise to be fulfilled
  */
 module.exports = function dispatchRequest(config) {
+  // Ensure headers exist
+  config.headers = config.headers || {};
+
+  // Transform request data
+  config.data = transformData(
+    config.data,
+    config.headers,
+    config.transformRequest
+  );
+
+  // Flatten headers
+  config.headers = utils.merge(
+    config.headers.common || {},
+    config.headers[config.method] || {},
+    config.headers || {}
+  );
+
+  utils.forEach(
+    ['delete', 'get', 'head', 'post', 'put', 'patch', 'common'],
+    function cleanHeaderConfig(method) {
+      delete config.headers[method];
+    }
+  );
+
   return new Promise(function executor(resolve, reject) {
     try {
       var adapter;