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
Revert "also eat keywords containing lowercase letters"

this was inadvertedly merged. the real fix is to make the parser
case-insensitive...
make the parser case-insensitive

i.e., make keywords ALL_UPPERCASE before using them because Eclipse
seems to be case-insensitive (although this is one of its undocumented
features...)