fix issue-1789

this PR fix issue 1789 and it allows to make expression against geometry fields, however imho it is more a workaround. It doesn't brake the backward compatibility, on the other hand I think that adding .st_astext() to all geometry fields by default is not the best way we can follow. 
The st_astext() converts the binary content of the database in a more readable form. However, the st_astext is not the only function we can use to convert the binary content (es in postgis we can use st_asewkt).
My view is to fix the issue with this PR, and review the whole code in a future release ex: by removing the explicit call to .st_astext, user can still add the .st_astext when needed.