Check if response status is unprocessableEntity (#31)

This PR adds a convenient helper to check for unprocessableEntity response status.

I bet we don't want to add a helper for each status code but this is motivated by the fact that Rails now returns 422 status code by default for form submission with error rails/rails#41026

So, we can legitimately expect doing this kind of check more often.

Author: olimart

src/fetch_response.js

@@ -19,6 +19,10 @@ export class FetchResponse {
     return this.statusCode === 401
   }
 
+  get unprocessableEntity () {
+    return this.statusCode === 422
+  }
+
   get authenticationURL () {
     return this.response.headers.get('WWW-Authenticate')
   }