Added enhanced toFormData implementation with additional options support; (#4704)

Updated default notation for arrays and objects to bracket style;
Added `multer/express.js` tests;
Updated README.md;

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

lib/defaults/index.js

@@ -77,7 +77,11 @@ var defaults = {
 
     if ((isFileList = utils.isFileList(data)) || (isObjectPayload && contentType === 'multipart/form-data')) {
       var _FormData = this.env && this.env.FormData;
-      return toFormData(isFileList ? {'files[]': data} : data, _FormData && new _FormData());
+      return toFormData(
+        isFileList ? {'files[]': data} : data,
+        _FormData && new _FormData(),
+        this.formSerializer
+      );
     } else if (isObjectPayload || contentType === 'application/json') {
       setContentTypeIfUnset(headers, 'application/json');
       return stringifySafely(data);