Added extensions for getting content header values

[jizc]

Description

I liked the new extensions added in #2221, but immediately wished for the same extensions for the ContentHeaders collection. This PR adds GetContentHeaderValue and GetContentHeaderValues extension methods.

I also removed the ? after HeaderParameter.Value properties in the extensions, as they are no longer needed after #2241.

Purpose

This pull request is a:

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[jizc]

Thank you for merging the PR 🎉