Prevent overriding port option of VirtualHost in HTTP checker

This patch prevents HTTP checker from overriding port option 
in VirtualHost string.

Host's port being specified in HTTP GET request 
only if `VirtualHost` option is not defined, otherwise used `VirtualHost` option itself.

For example if VirtualHost not defined and RealServer is example.com:80 then
HTTP GET Host will be like "Host: example.com:80".
But if VirtualHost is defined as virtual.com:8080 then 
HTTP GET Host will be like "virtual.com:8080". 
In current master branch the second situation will result in HTTP GET
Host like "Host: virtual.com:8080:80" which is incorrect.
This patch fixes this behaviour.