Performance issue by `supported_directives` method

[igrep]

Problem

The change in #179 can cause a performance issue.

I learned that lib/secure_headers/headers/content_security_policy.rb,
UserAgentParser.parse(@ua) in supported_directives is executed for each request.
As user_agent_parser's author is planning to remove, UserAgentParser.parse(@ua) is problematic.
Because it loads common regexps yaml file as large as 4.7k lines on every call.

Possible solution

According to user_agent_parser's README's recommendation, we should instantiate a user_agent_parser by UserAgentParser::Parser.new, then cache it.
So, which class / object should cache a user_agent_parser?

[oreoshake]

In the upcoming 3.x series I've moved it here: https://github.com/twitter/secureheaders/blob/dd1fd6ab751e6026431be9ecb25e8664f70ee714/lib/secure_headers.rb#L26. I'll do the same for the 2.x series. Thanks.

[oreoshake]

2.4.3 has been released with this fix. Thanks!

[igrep]

You're welcome! 😄
I confirmed it works perfectly on our production environment! 🎉
Thank you very much for fixing! 👍