Fixing content-type header repeated (#4745)

Co-authored-by: Jay <jasonsaayman@gmail.com>

lib/core/dispatchRequest.js

@@ -5,6 +5,7 @@ var transformData = require('./transformData');
 var isCancel = require('../cancel/isCancel');
 var defaults = require('../defaults');
 var CanceledError = require('../cancel/CanceledError');
+var normalizeHeaderName = require('../helpers/normalizeHeaderName');
 
 /**
  * Throws a `CanceledError` if cancellation has been requested.
@@ -40,6 +41,9 @@ module.exports = function dispatchRequest(config) {
     config.transformRequest
   );
 
+  normalizeHeaderName(config.headers, 'Accept');
+  normalizeHeaderName(config.headers, 'Content-Type');
+
   // Flatten headers
   config.headers = utils.merge(
     config.headers.common || {},