XML indentation with environment support

Complements #612 

This is a draft to get more suggestions. With this pull request I added a support to `indented` method to get a value from environment as discussed in #612.

Since this change affects `MockSerializationResult`, the constructor was deprecated and a new constructor with `Environment` parameter was added to keep compatibility backwards.

After this pull request, we can add the same support to JSON serialization.

Docs will improved before merge.
But if returns `false` if key wasn't found isn't enough, because if key not found returns `false` indented will be equals `false` always when key not found. But we need only if there is `false` in the properties file. Key not found we need to consider `indented=true`. I see `Environment.get` as the best choice.