Add User Agent without validation.

[alekw]

Description

Add User Agent without validation. It will allow usage of any string as User Agent. Useful for imitating other user agents.
Previous implementation has forbidden multiple characters, which are used in other http clients (not only .net).

#1943
#1905

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)

[dnfadmin]

All CLA requirements met.

[alexeyzimarev]

Thank you for the PR. Please remember to sign the CLA.

[alekw]

Thank you for the PR. Please remember to sign the CLA.

Yesterday I had some problems with signing CLA, today I have managed.