umrad/fields: Allow str as lhs in PathField filters.

This permits e.g. File.objects.filter(path='/some/path/here').
Previously it had to be path=Path('/some/path/here').

mibios/umrad/fields.py

@@ -193,6 +193,8 @@ def get_prep_value(self, value):
         elif self.root is None:
             return str(value)
         else:
+            if not isinstance(value, Path):
+                value = Path(value)
             return str(value.relative_to(self.root))
 
     def value_to_string(self, obj):