Tells whether or not this charset contains the given charset.
A charset C is said to contain a charset D if,
and only if, every character representable in D is also
representable in C. If this relationship holds then it is
guaranteed that every string that can be encoded in D can also be
encoded in C without performing any replacements.
That C contains D does not imply that each character
representable in C by a particular byte sequence is represented
in D by the same byte sequence, although sometimes this is the
case.
Every charset contains itself.
This method computes an approximation of the containment relation:
If it returns true then the given charset is known to be
contained by this charset; if it returns false, however, then
it is not necessarily the case that the given charset is not contained
in this charset.