Bump shelf to ca731ab09eba26d6e301fbc882a65621cb3cfed4
Bump shelf to ca731ab09eba26d6e301fbc882a65621cb3cfed4

Changes:
```
> git log --format="%C(auto) %h %s" 6f80ea1..ca731ab
 https://dart.googlesource.com/shelf.git/+/ca731ab Add a poweredByHeader argument to server (#272)

```

Diff: https://dart.googlesource.com/shelf.git/+/6f80ea14fcd13c911bd31d32260bf1f97ad799b1~..ca731ab09eba26d6e301fbc882a65621cb3cfed4/
Change-Id: I6f828214c084591ca5e9827ab2f6de07b626fbbe
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/259107
Commit-Queue: Nate Bosch <dc212c74d8afde9c690fdd09e74d3e5c5d5a3de0@google.com>
Reviewed-by: Kevin Moore <1d6e7d10d051cf619c5282da3c28fd8d3e0a65a4@google.com>
Commit-Queue: Kevin Moore <1d6e7d10d051cf619c5282da3c28fd8d3e0a65a4@google.com>
Auto-Submit: Nate Bosch <dc212c74d8afde9c690fdd09e74d3e5c5d5a3de0@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk