Patches UA and Proxy Settings in metsrv when running migrate

Earlier today, I saw that my User-Agent was changing to "METERPRETER_UA" after running "migrate" with windows/meterpreter/reverse_http.  It appeared that metsrv was not getting patched with MeterpreterUserAgent so I pulled the pertinent code (added the proxy stuff for completeness) from msf/core/handler/reverse_http.rb and dumped it here.