Make USER_AGENT parser writable

[george-gca]

I am trying to fetch a css file from Google fonts, and the information it generates depends on the user agent. When accessing the file from Google Chrome it returns all fonts inside as woff2 files, but when reading the file straight with load_uri it returns all fonts as ttf files. I believe being able to set the USER_AGENT value would fix this.

[grosser]

PR welcome, sounds like a small/innocent change

[george-gca]

What do you think would be the best solution for this? Create as an option in the constructor, or enable a setter?

[grosser]

lots of options in the constructer already so should be a good fit