[ExpressionLanguage] added default value in parse method

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

I was trying to follow doc instructions from here : http://symfony.com/doc/2.5/components/expression_language/caching.html#using-parsed-and-serialized-expressions.