fix addRowData with pos="after" with subgtid 
fix addRowData with pos="after" with subgtid 

See the [bug report](http://www.trirand.com/blog/?page_id=393/bugs/bug-in-addrowdata-method) for more details.

Additionally the code was changed to use `push` method of `Array` to add an element at the end of the array.
javapark/jqgrid