add custom channel to adunit

[sorry chou]

according to reference https://developers.google.com/adsense/host/v4.1/accounts/adunits/getAdCode

Optional parameters
hostCustomChannelId	string	Host custom channel to attach to the ad code.

but when using php client libraries, this parameter should be an array as follows

array('hostCustomChannelId', array('ca-host-pub-xxxx', 'xxxxx', ....))

:)