Fix for Fix for 151: Column searchablity can offset the column type index

In function _fnBuildSearchRow(), aData[idx] incorrectly got investigated even if aoColumns[i].bSearchable was false.

(If you're reviewing this commit, imagine that aoColumns[i].length ==
10 and aData.length == 1 and the first and only i where
aoColumns[i].bSearchable is true is i== 5. You'll notice that e.g.
aData[8] also gets investigated)

History: First there was this commit:

    commit 065c2cc66b872c6964b7d31f12941f23e7f1f171
    Date:   Thu Nov 1 21:45:48 2012 +0000

        Fix: Filtering wasn't correctly applying the `type` adjustments needed for the global filter. For example this meant that html was not stripped from 'html' type columns, resulting in filtering being done on html tags/attributes as well as the content.

It fixed e.g. this issue:
"How to search only non html content - DataTables forums"
http://www.datatables.net/forums/discussion/13667/how-to-search-only-non-html-content/p1

However, it was buggy, and then this got committed.

    commit f3ce2e2d44db8eb43750435c55eb2551b2c6019e
    Date:   Sat Feb 16 11:27:41 2013 +0000

        Fix 151: Column type was being offset when columns were not searchable

However, that requires this commit to work properly.