Fix(workaround really) the bogus addition of user-agent:

[ldemailly]

by go client lib when none is set in a proxy context

fixes #15

before:

make dev # in proxy
go run . curl -loglevel debug -H user-agent: -H foo:bar localhost:8001/debug  # in Fortio with Fortio server running
[...]
User-Agent: Go-http-client/1.1
X-Forwarded-For: 127.0.0.1

after

go run . curl -loglevel debug -H user-agent: -H foo:bar localhost:8001/debug 
[...]
headers:

Host: localhost:8001
Accept-Encoding: gzip
Foo: bar
X-Forwarded-For: 127.0.0.1