Set a default User-Agent header when making Request objects

[akhra]

An ops-minded coworker pointed out that this was missing from my Magicbane-powered requests. There should be some default, since a lot of APIs will reject any request without one.

[valpackett]

hah wow you're actually using this client :)

We'll need some actual "set header" functionality in that case… e.g. currently the form/json functions replace all headers with Content-Type. (Wait, maybe a User-Agent is already present on defaultRequest and this is what removes it?)

[akhra]

Ah, that's very likely -- I'm sure I remember there being a default set when I used http-conduit a few projects back. I've got a function in my local codebase that's roughly union newHeaders existingHeaders, I'll see about making it generic and sending a PR tomorrow.

[akhra]

Unfortunately there is no User-Agent in defaultRequest. But #14 makes it far easier to add one downstream, and also prevents postForm/postJson clobbering headers added before their use.

I didn't directly address this for now, to avoid messing with style/structure too much -- e.g. I'm not sure if having unqualified export lists is a priority -- and also I have no idea what the agent string should be.