Make capitalize lowercase rest of the string

It was unexpected, that `.capitalize` doesn't lowercase rest of the word. Checked in Ruby, Python, and Clojure - all have the same semantic - rest of the word is lowercased:

    [1] pry(main)> "MiXeD cAsE".capitalize
    => "Mixed case"

    >>> "MiXeD cAsE".capitalize()
    'Mixed case'

    user=> (clojure.string/capitalize "MiXeD cAsE")
    "Mixed case"

Would be cool to have consistent with other behavior in `underscore.string`.