WebSocket: more validation and error reporting

[graebm]

Update aws-c-http submodule, to get various WebSocket improvements:

• https://github.com/awslabs/aws-c-http/pull/410
• https://github.com/awslabs/aws-c-http/pull/415
• https://github.com/awslabs/aws-c-http/pull/416
• https://github.com/awslabs/aws-c-http/pull/417
• https://github.com/awslabs/aws-c-http/pull/418

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[graebm]

Closing. These changes were already merged in by another PR...