get_primary_key() works with inheritance and multpiple pks

* `contrib.sqla.tools.get_primary_key()` now works with inhteritance *and* multiple primary keys.

Every primary key is checked if it is a inherited one, and if it's inherited, the *current* column.key is returned. I *think* it should be safe just to use *prop.expression* here, but I am not absolutely sure.

( see http://docs.sqlalchemy.org/en/rel_0_8/orm/internals.html?highlight=columnproperty#sqlalchemy.orm.properties.ColumnProperty.expression )

flask_admin/contrib/sqla/tools.py

@@ -21,10 +21,12 @@ def get_primary_key(model):
 
     pks = []
     for p in props:
-        if hasattr(p, 'columns'):
-            for c in p.columns:
-                if c.primary_key:
-                    pks.append(c.key)
+        if hasattr(p, 'expression'):    # expression = primary column or expression for this ColumnProperty
+            if p.expression.primary_key:
+                if is_inherited_primary_key(p):
+                    pks.append(get_column_for_current_model(p).key)
+                else:
+                    pks.append(p.columns[0].key)
     if len(pks) == 1:
         return pks[0]
     elif len(pks) > 1: