client: add default user agent

[Fishrock123]

As suggested in discord, I think.

[Fishrock123]

Note: the tests for this are brittle. The order of added header is non-deterministic (HashMap iterator).

[jbr]

Seems reasonable to sort headers by either addition order or alphabetically

[Fishrock123]

The problem is we are comparing to raw output files and unless headers are always going to be sorted there isn't a way to do that just for test output without re-writing the tests.