tests: branch coverage in _utils.py, middleware/gzip.py, routing.py and requests.py

[lealre]

Summary

Related to Issue #2452

This PR covers all 9 missing cases.

starlette/_utils.py 78–82

Extend the # pragma: no cover to include the condition.

starlette/middleware/gzip.py 16–22, 91–end

Related to conditional cases with scope type and message type.
Added # pragma: no branch to all cases.

starlette/routing.py 199–202, 401–430, 484–496

Related to conditional cases where the scope type is not 'http' or 'websocket'.
Added # pragma: no branch to all cases.

starlette/requests.py 114–116, 233–225, 253–278

Although it's from requests.py, the test related to lines 114–116 was added in test_routing.py.
For the other two cases, # pragma: no branch was added.

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.