[FIX] web: mockReadGroup does not mix 'false' and false

An implicit call to toString was made for the
groupbys values in the function used to
group records in _mockReadGroup.
Consequently, the values false and 'false' were
mixed and the corresponding records put in the
same group.

addons/web/static/tests/helpers/mock_server.js

@@ -940,7 +940,7 @@ var MockServer = Class.extend({
                 if (fields[fieldName].type === 'date') {
                     value += formatValue(groupByField, record[fieldName]);
                 } else {
-                    value += record[groupByField];
+                    value += JSON.stringify(record[groupByField]);
                 }
             });
             return value;