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.
As I remember:
```
if user don't call indented, indented = false
if user call indented
	and environment = false, indented = false
	and environment = true, indented = true
	and environment is undefined, indented = true
```