Use "str2bool_i" instead of "== 'true'"

Generated with:

grep -ril "== 'true'" puppet | xargs sed -i -e "s/map_params('\([^']*\)') == 'true'/str2bool_i(map_params('\1'))/"
Merge pull request #341 from jistr/str2bool_i

[when convenient] Use "str2bool_i" instead of "== 'true'"