Do not send keep-alive or close in HTTP Upgrade requests (#732)

Do not send keep-alive or close in HTTP Upgrade requests (#732)

A presence of a Connection:keep-alive or Connection:close header in an
Upgrade request sent by Squid breaks some Google Voice services. FWIW,
these headers are not present in RFC 7230 Upgrade example, and popular
client requests do not send them.

* In most cases, Squid was sending Connection:keep-alive which is
  redundant in Upgrade requests (because they are HTTP/1.1).

* In rare cases (e.g., server_persistent_connections=off), Squid was
  sending Connection:close. Since we cannot send that header and remain
  compatible with popular Upgrade-dependent services, we now do not send
  it but treat the connection as non-persistent if the server does not
  upgrade and expects us to continue reusing the connection.