bug - extract datetime lowercases remainder

[JarbasAl]

the remainder text should not be lowercased, this impacts comparisons between remainder and original utterance

similar to #145 which has been fixed in #146

will fix this in the datetime rewrite #74

[ChanceNCounter]

I think this can be addressed without much of a performance hit by way of rolling token replacement. The alternative, iterating over the original and replacing only the chunks where this_chunk != original.lower(), obviously doesn't scale well to large inputs.