Fix broken boolean validator

The boolean validator treats missing values as True. They should be treated as False, both semantically and because unchecked  HTML checkboxes in forms transmit empty values on form submits

ckan/logic/validators.py

@@ -55,6 +55,8 @@ def int_validator(value, context):
         raise Invalid(_('Invalid integer'))
 
 def boolean_validator(value, context):
+    if value is missing:
+        return False
     if isinstance(value, bool):
         return value
     if value.lower() in ['true', 'yes', 't', 'y', '1']: