Fix juce::String::natureStringCompare

Fixes juce::String::compareNatural behaviour: it is case sensitive when characters differ.
This doesn't match the description and means it's not a strict weak ordering and so not useful for sorting.

Consider the following:

assert(juce::String("a").compareNatural("A") == 0);
assert(juce::String("A").compareNatural("B") < 0);
assert(juce::String("a").compareNatural("B") < 0); // Fails