helm-do-ag now respects variable helm-ag-thing-at-point

When helm-ag-thing-at-point is set, helm-do-ag currently does not respect the variable.
This patch changes that.
This entails that when the user has their cursor above, e.g., a symbol, and helm-ag-thing-at-point is set to symbol, the helm input buffer will be pre-populated with the symbol beneath the cursor.