[FIX] models: do not prefetch fields while the model schema is created

The following case has shown the issue: extend the model `res.company` by
adding at least two fields F and G, where F has a default value defined as:

    lambda self: self.env.user.company_id.name

If the column F is created before G in the database, the existing records will
be filled with the default value of F.  When the default value is computed, the
field `name` from a `res.company` is read, and other fields are prefetched,
including G.  This operation fails, because G does not exist in database yet!

openerp/models.py

@@ -2422,8 +2422,12 @@ class BaseModel(object):
         """
         self._foreign_keys = set()
         raise_on_invalid_object_name(self._name)
-        if context is None:
-            context = {}
+
+        # This prevents anything called by this method (in particular default
+        # values) from prefetching a field for which the corresponding column
+        # has not been added in database yet!
+        context = dict(context or {}, prefetch_fields=False)
+
         store_compute = False
         stored_fields = []              # new-style stored fields with compute
         todo_end = []