Don't append port to Host header if not required

This is possibly a an issue relating more to misconfiguration of remote servers, however a couple of SOAP servers I wish to use don't correctly respond with WSDL if the port is appended to the `Host` header in the http request. This is what `node-soap@0.5.0` does currently where previous versions did not.

Since this is optional for HTTP requests, this PR changes the current behaviour by not appending `:80` to the `Host` header. 

If this is not a desirable solution, possibly an option could be added to account for this behaviour?