Lowercase RDATA dnames
All dnames in RDATA are converted to lowercase when adding to the zone. There were two possible solutions to this. Description and argumentation are in #290 (closed). This, however, does not remove now possibly redundant conversions to lowercase all over the code. That may be done as a next step.
Also added some tests testing potential problems related to case in RDATA dnames.
Fixes #290 (closed)