Ensure camelize respects first_letter_in_uppercase

I have encountered an inconsistency in the `camelize()` method.

If it is passed a string with an UppercaseFirstLetter but the second argument `first_letter_in_uppercase` is passed in as `false`, result is returned without ensuring the first letter is lower-cased.

Although the main use case appears to be changing underscore_words to camelCase, I am using it to convert paths (e.g. `My/Path/To/Something` should become `myPathToSomething`) in which case the first word is not ensured to start with a lowercase letter.

This changeset fixes the issue, and I've added this to the test suite.

Hope you accept the change & update npm soon.

Thanks,
Jed.