fix: Enforce Locale.US for AwsRequestSignerTest

[jmahonin]

Enforce Locale.US for AwsRequestSignerTest. Date parsing can fail in other locales.

Specifically, in en-CA (Canada), the tests's defined date Mon, 09 Sep 2011 23:36:00 GMT won't parse correctly, as the weekday short name for the locale is Mon. (extra period).

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes #1110 ☕️

If you write sample code, please follow the samples format.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[conventional-commit-lint-gcf]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot
https://conventionalcommits.org/

[jmahonin]

CLA was signed yesterday for a workgroup that jmahonin@decodable.co is a member of. Maybe there's a propagation delay?
CLA verification succeeded

[aeitzman]

LGTM