Returns a new string containing the downcased characters in +self+:

   'Hello, World!'.downcase      # => "hello, world!"
   'ТЕСТ'.downcase               # => "тест"
   'よろしくお願いします'.downcase # => "よろしくお願いします"

Some characters do not have upcased and downcased versions.

The casing may be affected by the given +mapping+;
see {Case Mapping}[rdoc-ref:case_mapping.rdoc].

Related: see {Converting to New String}[rdoc-ref:String@Converting+to+New+String].
