fix crash in updateIndex when last doc is a design doc

The conditional that skips indexing design docs advances the cursor
without updating the while loop condition (`keepGoing`). If the last row
in the cursor is a design doc, this will cause the next loop iteration
to read past the end of the cursor and throw a
CursorIndexOutOfBoundsException.
Merge pull request #173 from mjq/index_crash

Fix crash in updateIndex when last doc is a design doc