Set User-Agent header in transport

[daenney]

Currently requests set their own User-Agent. Set it in the transport instead, unless it's been explicitly set on the request. This retains the flexibility of tweaking the UA if necessary, without having to remember to set it.

[daenney]

One thing I noticed is that the code used to do req.Headers.Add() for the UA instead of .Set(). This could theoretically lead to the UA header being multi-valued, but I don't think that header is allowed/specified as list-based or multi-valued. It's why I opted for checking if the header is unset in req and going with .Set() to make that more explicit. Given we're always the ones who build the req this doesn't make a difference in practice.