Add User-Agent to download recipe for sparkle feeds

[andrewzirkel]

Is your feature request related to a problem? Please describe.
When RecipeRobot accesses a sparkle feed it does so with a user-agent. When the recipe is written though there is no user-agent passed to the SparkleUpdateInfoProvider processor. This could (and did for me) cause autopkg to fail if the sparkle provider requires a user agent.

Describe the solution you'd like
Embed the same user-agent that RecipeRobot is using to access the sparkle feed in the download recipe as a argument to the SparkleUpdateInfoProvider Processor.

Additional context
This is the feed I was working on: https://www.kutasoftware.com/updates/appcast_a1m.xml
When the url is accessed via curl with no user-agent a 406 error is returned.