input: fix scmOverrides regression

The "set" property of an scmOverrides is a map of keys and their new
values. Commit 790c1be accidentally changed its schema so that it
accepted anything instead of only a map of strings to arbitrary values.

Fixes #497.

pym/bob/input.py

@@ -3139,7 +3139,7 @@ def updateWhiteList(x):
                     schema.Optional('if') : schema.Or(str, IfExpression),
                     schema.Optional('match') : schema.Schema({ str: object }),
                     schema.Optional('del') : [str],
-                    schema.Optional('set') : object,
+                    schema.Optional('set') : schema.Schema({ str: object }),
                     schema.Optional('replace') : schema.Schema({
                         str : schema.Schema({
                             'pattern' : str,