Modify response- and request-body snippets to have language attribute depending on `Content-Type` header

Current default snippet
```
[source,options="nowrap"]
----
{{body}}
----
```

It's much more convenient to automatically add json attribute like this:
```
[source,{{language}},options="nowrap"]
----
{{body}}
----
```

`AbstractBodySnippet#createModel` could set `language` property in model depending on the `contentType`