fix: remove deletion of Content-Type from headers

lib/adapters/xhr.js

@@ -15,10 +15,6 @@ module.exports = function xhrAdapter(config) {
     var requestHeaders = config.headers;
     var responseType = config.responseType;
 
-    if (utils.isFormData(requestData)) {
-      delete requestHeaders['Content-Type']; // Let the browser set it
-    }
-
     var request = new XMLHttpRequest();
 
     // HTTP basic authentication