Add independent maxBodyLength option (#2781)

* Add independent option to set the maximum size of the request body

* Remove maxBodyLength check

* Update README

* Assert for error code and message

lib/defaults.js

@@ -74,6 +74,7 @@ var defaults = {
   xsrfHeaderName: 'X-XSRF-TOKEN',
 
   maxContentLength: -1,
+  maxBodyLength: -1,
 
   validateStatus: function validateStatus(status) {
     return status >= 200 && status < 300;