Fixes an issue in the exists and toSql function

Update to correct a bug when using the exists function within a Model object. 
Example: when using User::where('id', '>', '0')->exists() it was returning a Builder object. With this change we can know get the boolean value as expected.
The same issue was happening with the toSql function.
Merge pull request #1054 from miguelbgouveia/patch-2

Fixes an issue in the exists and toSql function