Use ETag from headers when parsing If-Range in FileResponse

[viccie30]

Summary

FileResponse.set_stat_headers() takes care not to overwrite any headers set on the response, but FileResponse._should_use_range() ignores any set headers and unconditionally uses its own ETag and Last-Modified calculation. This commit rectifies that.

I have chosen to code self.headers[HEADER] instead of self.headers.get(HEADER), because FileResponse.__call__() always calls set_stat_headers() and therefore the headers should always be set.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.