case sensitive sorting of local data

Hi Tony,

event with the option `ignoreCase: false` local sorting of grid is not case sensitive now. After the suggestion changes the problem will be fixed. Additionally I changed the case of strings used during comparing from low case to upper case and removed `toLowerCase` call from `_compare` which should improve the performance a little.

[The demo](http://www.ok-soft-gmbh.com/jqGrid/CaseSensitiveSorting0.htm) displays the following results:

<img src="http://www.ok-soft-gmbh.com/jqGrid/CaseSensitiveSorting0.png" alt="bug" />

[Another demo](http://www.ok-soft-gmbh.com/jqGrid/CaseSensitiveSorting.htm), which use the fixed code, display as the results the following:

<img src="http://www.ok-soft-gmbh.com/jqGrid/CaseSensitiveSorting.png" alt="fix" />

Best regards
Oleg