fix($http): do not throw error if `Content-Type` is not `application/json` but response is JSON-like

Previously, when the response data was JSON-like, `$http` would try to
`JSON.parse` them and throw if they were not actually JSON. This happened even
if the `Content-Type` header was not `application/json`. As a result, it was not
possible to send `text/plain` data that looked like JSON (e.g. `{abcd}`).
The reason for not relying solely on the `Content-Type` is that many users serve
JSON data without proper headers.

This commit fixes it by returning the raw response text if `$http` fails to
parse a JSON-like response, unless the `Content-Type` header has been explicitly
set to `application/json` (in which case it will still fail with an error).

Fixes #16027

Closes #16075