[Serializer] Add the missing context support inside the XmlEncoder

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

$context variable was added in symfony 2.3 but not inside this encoder