Bug fix: peer verification fails when using TLS over an HTTP proxy

[dariushoule-stripe]

When tunneling a TLS connection through an HTTP proxy the hostname of the proxy server is incorrectly used for peer verification. As-is the following error will be raised:

em-http-request-1.1.7/lib/em-http/http_connection.rb:72:in `ssl_handshake_completed': host "some.proxy.server.tld" does not match the server certificate (OpenSSL::SSL::SSLError)

It seems that this is because connopts.host will always contain the proxy hostname when a proxy is configured and is unconditionally used for peer verification.

The resolution suggested in this PR is instead using @tls[:sni_hostname] which will contain the correct downstream hostname.