JSON body is not parsed when Content-Type contains a charset

My back-end returns JSON data with a content-header that contains the suffix `+json`'and the `charset=utf-8'`
The content-type is not recognized as JSON and the body is returned as a plain string instead of JSON.

The problem is the way that the `Content-Type` header is parsed:
`Content-Type: application/whatever+json;charset=utf-8`

This is the method for parsing the header (and the body):
```
protected parseBody(response: FetcherResponse): Promise {
const contentType = response.headers.get('Content-Type');
const contentLength = response.headers.get('Content-Length');
if (
// As one might expect, a "204 No Content" is empty! This means there
// isn't enough to `JSON.parse`, and trying will result in an error.
response.status !== 204 &&
contentLength !== '0' &&
contentType &&
(contentType.startsWith('application/json') ||
contentType.endsWith('+json'))
) {
return response.json();
} else {
return response.text();
}
}

```

The condition for checking on JSON fails, because:
`contentType.endsWith('+json')`
is `false` in the case that there is a charset defined at the end of the header

Start by locating the parseBody entry point shown in the issue and inspect how Content-Type is checked before response.json() or response.text(). Verify the fix with a response using application/whatever+json;charset=utf-8; done means that JSON body is parsed rather than returned as a plain string.

Written by the indexing model from the issue text.