Fixing accept header normalization (#1698)

Fixes override behavior for `accept` header, when case does not match
the one used in defaults.js

lib/defaults.js

@@ -30,6 +30,7 @@ var defaults = {
   adapter: getDefaultAdapter(),
 
   transformRequest: [function transformRequest(data, headers) {
+    normalizeHeaderName(headers, 'Accept');
     normalizeHeaderName(headers, 'Content-Type');
     if (utils.isFormData(data) ||
       utils.isArrayBuffer(data) ||