fix for application/json stats

[mattjackson220]

What does this PR (Pull Request) do?

• This PR is not related to any Issue

This PR adds application/json as a valid content type for stats in Traffic Monitor

Which Traffic Control components are affected by this PR?

• Traffic Monitor

What is the best way to verify this PR?

Verify all TM unit tests pass, verify that application/json is a valid content type when getting astats info from an edge or mid cache

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

The following criteria are ALL met by this PR

This PR does not need docs or changelog updates since its just adding another content type

• This PR includes tests
• I have explained why documentation is unnecessary
• An update to CHANGELOG.md is not necessary
• This PR includes any and all required license headers
• This PR ensures that database migration sequence is correct OR 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

[ezelkow1]

LGTM