Update reference to user_agent to agent

[Mpdreamz]

Since I don't see user_agent mentioned elsewhere I presumed this to be agent.

[ruflin]

user_agent was temporarly removed for beta1: #172 It's not the same as agent.

[webmat]

Haha we missed removing this mention of user_agent :-) As Nic said, it's only been removed temporarily.

I'll close this PR as it will most likely come back as user_agent. We've just taken it out for this first official release, with the goal of reviewing the field names in the user_agent field set.