added cell type = formula check when looping through the shared formula range, to ignore any non-formula cells.
Also refactored a bit to enable passing in the evaluation context, as getCellFormula() uses it behind the scenes when evaluating a shared formula cell (has to shift the formula references based on the master cell). Review of these changes is welcome, as always.
Checked all other code referencing the "SHARED" enum, and didn't see anything else that dealt with formula cell values and thus would need to notice non-formula cells.
Added unit test based on the failing file from Bugzilla. Test failed until the fixed code was in place.
git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1844295 13f79535-47bb-0310-9956-ffa450edef68
