also eat keywords containing lowercase letters

The Norne deck actually exhibits this atrocity in form of the
'fluxnum' keyword in the file 'INCLUDE/PETRO/FLUXNUM_0704.prop'.

I don't know if Eclipse cares about the case of the keywords, but
opm-parser currently does for sure. (If Eclipse turns out to be
case-insensitive, the easiest fix for us is to just make all keywords
ALL_UPPERCASE...)
Merge pull request #351 from joakim-hove/cherry-pick-339

Cherry pick from #339