TR: Send reason-phrase for HTTP API responses

[zrhoffman]

What does this PR (Pull Request) do?

Reason-phrases are sent for routing over HTTP, routing over HTTPS, and the API over HTTPS, but not for the API over HTTP. This PR adds reason-phrases for the API over HTTP.

As @ocket8888 noted in #4074, Pygments needs reason-phrases for HTTP responses (of any HTTP version).

• This PR is not related to any Issue

This PR does not include tests because

• The only component that depends on reason-phrases as far as I know is the documentation.
• HTTP/2 responses have no reason-phrases.
• If documentation ever stops failing to build due to no reason-phrase in a documented HTTP response, there is no reason to send one anymore, so building the documentation is a de facto test.

Which Traffic Control components are affected by this PR?

• Traffic Router

What is the best way to verify this PR?

In the CDN-in-a-box directory,

docker-compose exec trafficrouter curl -I http://trafficrouter:3333/crs/stats

If this is a bug fix, what versions of Traffic Control are affected?

• master (1c2ebee)
• 3.1.0

The following criteria are ALL met by this PR

• I have explained why tests are unnecessary
• This PR does not need documentation because expected behavior does not change
• This PR does not include an update to CHANGELOG.md, such an update is not necessary
• This PR includes any and all required license headers
• This PR does not include a database migration
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

Additional Information

[asf-ci]

Can one of the admins verify this patch?

[zrhoffman]

We're using Tomcat 9, which removes reason-phrases altogether, so #4096 is no longer needed.