Handler breaks custom user-agent

[andreymal]

Without this:

>>> from urllib.request import *
>>> r = Request('http://localhost:8989/')
>>> r.add_header('User-Agent', 'foo')
>>> urlopen(r)

GET / HTTP/1.1
Accept-Encoding: identity
Connection: close
Host: localhost:8989
User-Agent: foo

With this:

>>> from urllib.request import *
>>> from keepalive import HTTPHandler
>>> opener = build_opener(HTTPHandler())
>>> install_opener(opener)
>>> r = Request('http://localhost:8989/')
>>> r.add_header('User-Agent', 'foo')
>>> urlopen(r)

GET / HTTP/1.1
Host: localhost:8989
Accept-Encoding: identity
User-agent: Python-urllib/3.5  # <--- WAT
User-agent: foo

[wikier]

There is no custom User-Agent handling in keepalive, @andreymal; so I don't know where/when urllib could be adding that header. I'll take a look anyway.