Make the apply_inflections method case-sensitive

Since d3071db1, the apply_inflections method check if the downcased
version of a string is contained inside the "whitelist" of uncountable
words. However, if the word is composed of capital letters, it won't be
matched in the list while it should.

We can't simply revert to the previous behavior as there is a
performance concern (benchmarked over /usr/share/dict/words):

Before d3071db1  135.610000   0.290000  135.900000 (137.807081)
Since d3071db1   22.170000    0.020000  22.190000  ( 22.530005)
With the patch   25.130000    0.070000  25.200000  ( 25.611538)

Benchmarked with http://git.io/aFnWig