Automatic merge from submit-queue (batch tested with PRs 49081, 49318, 49219, 48989, 48486)

Use case-insensitive header keys for `--requestheader-group-headers`.

This flag is documented as being case-insensitive, but the code was
doing a case-sensitive map lookup.

**Release note**:
```release-note
Bug fix: Parsing of `--requestheader-group-headers` in requests should be case-insensitive.
```