Added support for encoding and decoding namespaced xml (xmlns)

Q | A
------------ | -------------
Bug fix? | no
New feature? | yes
BC breaks? | no
Deprecations? | no
Tests pass? | yes
Fixed tickets | #10625
License | MIT

Complete this PR : https://github.com/symfony/symfony/pull/9156
Isset is unnecessary... Unset will work without warning if the key is defined or not