[Serializer] Added a test for decoding xml with whitespaces

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

The PR https://github.com/symfony/symfony/commit/466a3d70b7972dbac20264c254351ca31ee5c476 introduced a BC break because white spaces were previously decoded as text nodes.

So this PR reverts it and adds a context option to configure it.