Be more lenient when parsing "true" and "false" in tag-value

[armintaenzertng]

In #753 the case-sensitivity in regards to the values "true" and "false" in tag-value have been discussed.
It was decided that these should be parsed strictly with regard to the casing. I have changed my mind since then and now strongly propose to be more lenient and allow other casings, too.

We could log a warning when the provided document does not adhere to the "true" lower-casing convention.