Fix GroupByParam to return original param string

Fix GroupByParam to return original param string

Page.GroupByParam function internally uses Page.GetParam to get a
parameter value for a key of a page group but now Page.GetParam returns
a lowercase character string every time. It has no need to using
lowercase character string as a group key value and it confuse a
function user.

This fixes it to keep and return an original parameter string as a group
key value.

Fix #1564