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 <[email protected]>
Reviewed-by: Kevin Moore <[email protected]>
Commit-Queue: Kevin Moore <[email protected]>
Auto-Submit: Nate Bosch <[email protected]>

Author: natebosch

DEPS

@@ -139,7 +139,7 @@ vars = {
   "pub_rev": "ec35d46261b610e558dfd0d8525ca3fc8387b4b7", # manually rev'd
   "pub_semver_rev": "9fd28757ba45961ac5449e0f2b0020670e921475",
   "root_certificates_rev": "692f6d6488af68e0121317a9c2c9eb393eb0ee50",
-  "shelf_rev": "6f80ea14fcd13c911bd31d32260bf1f97ad799b1",
+  "shelf_rev": "ca731ab09eba26d6e301fbc882a65621cb3cfed4",
   "source_map_stack_trace_rev": "72dbf21a33293b2b8434d0a9751e36f9463981ac",
   "source_maps_rev": "e93565b43a7b6b367789de8ffba969c4ebeeb317",
   "source_span_rev": "ff03af16474ce91c89eb3bc28182866f4cb6dfb0",