Add and apply Rails/ResponseParsedBody rubocop rule

This rule was introduced in rubocop-rails 2.18.0.

Since using `response.parsed_body` is shorter than using
`JSON.parse(response.body)`, this also means we can group some lines in
one.

.rubocop.yml

@@ -408,6 +408,9 @@ Rails/RelativeDateConstant:
 Rails/RequestReferer:
   Enabled: true
 
+Rails/ResponseParsedBody:
+  Enabled: true
+
 Rails/ReversibleMigration:
   Enabled: true