cannot reload trytond_gis dependant module
I believe the "type" at line 50 in table.py should be "type_"
Here is the end of the stack trace I obtain when I reload a module containing a model with a geometry column:
```
...
File "/usr/local/lib/python3.5/dist-packages/trytond_gis/postgis/table.py", line 22, in add_column
column_adder(column_name, abstract_type, default, comment)
File "/usr/local/lib/python3.5/dist-packages/trytond_gis/postgis/table.py", line 50, in add_geometry_column
if prev_type.upper() != type.upper():
AttributeError: type object 'type' has no attribute 'upper'
```