Release/0.21.4 (#4025)

* fix json transform when data is pre-stringified (#4020)

* [Updating] incorrect JSON syntax in README.md

* [Releasing] v0.21.4

Co-authored-by: Guillaume FORTAINE <guillaume+github@fortaine.com>

lib/defaults.js

@@ -26,6 +26,21 @@ function getDefaultAdapter() {
   return adapter;
 }
 
+function stringifySafely(rawValue, parser, encoder) {
+  if (utils.isString(rawValue)) {
+    try {
+      (parser || JSON.parse)(rawValue);
+      return utils.trim(rawValue);
+    } catch (e) {
+      if (e.name !== 'SyntaxError') {
+        throw e;
+      }
+    }
+  }
+
+  return (encoder || JSON.stringify)(rawValue);
+}
+
 var defaults = {
 
   transitional: {
@@ -58,7 +73,7 @@ var defaults = {
     }
     if (utils.isObject(data) || (headers && headers['Content-Type'] === 'application/json')) {
       setContentTypeIfUnset(headers, 'application/json');
-      return JSON.stringify(data);
+      return stringifySafely(data);
     }
     return data;
   }],