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   22.060000    0.020000  22.080000  ( 22.125771)

Benchmarked with http://git.io/aFnWig

This way, the solution is to put the down-case version of words inside
the @uncountables array.
Merge pull request #14247 from robin850/inflections-with-uncountables

Make the apply_inflections method case-sensitive
rename sequence only if it exists