XML now have indented option

Closes #612.

Like Gson serialization, XML now is serialized without indentation by default. And an option to indented was added.

Note: after this pull request, I can add option to get from environment properties.
I think that instead of create a method `indentedIf(EnvironmentType)` we can simple change this method to something like this:

`indented = environment.get("the-property", "true").equals("true")`