fix(headers): fixed common Content-Type header merging; (#5832)

lib/core/Axios.js

@@ -73,15 +73,13 @@ class Axios {
     // Set config.method
     config.method = (config.method || this.defaults.method || 'get').toLowerCase();
 
-    let contextHeaders;
-
     // Flatten headers
-    contextHeaders = headers && utils.merge(
+    let contextHeaders = headers && utils.merge(
       headers.common,
       headers[config.method]
     );
 
-    contextHeaders && utils.forEach(
+    headers && utils.forEach(
       ['delete', 'get', 'head', 'post', 'put', 'patch', 'common'],
       (method) => {
         delete headers[method];

lib/defaults/index.js

@@ -8,10 +8,6 @@ import toURLEncodedForm from '../helpers/toURLEncodedForm.js';
 import platform from '../platform/index.js';
 import formDataToJSON from '../helpers/formDataToJSON.js';
 
-const DEFAULT_CONTENT_TYPE = {
-  'Content-Type': undefined
-};
-
 /**
  * It takes a string, tries to parse it, and if it fails, it returns the stringified version
  * of the input
@@ -150,17 +146,14 @@ const defaults = {
 
   headers: {
     common: {
-      'Accept': 'application/json, text/plain, */*'
+      'Accept': 'application/json, text/plain, */*',
+      'Content-Type': undefined
     }
   }
 };
 
-utils.forEach(['delete', 'get', 'head'], function forEachMethodNoData(method) {
+utils.forEach(['delete', 'get', 'head', 'post', 'put', 'patch'], (method) => {
   defaults.headers[method] = {};
 });
 
-utils.forEach(['post', 'put', 'patch'], function forEachMethodWithData(method) {
-  defaults.headers[method] = utils.merge(DEFAULT_CONTENT_TYPE);
-});
-
 export default defaults;