setItemDictionaryFlag() and setItemDictionaryFlag() disallow capital letters

Previously capital letters were allowed, but this is no longer the case. They're not troublesome characters like periods, so removing them is not necessary.

This is also breaking change for any modules that rely on capitalized keys.

The current behaviour could be accepted as the new standard however, so solving this is not strictly necessary.

The underlying cause is String.slugify() being used to sanitize the keys, which works perfectly, except for it also disallowing capital letters.