Reported by Remi Forax <###@###.###>

Severals methods that works with StringBuffer have no
StringBuilder equivalent.

1) There is no contentEquals(StringBuilder) in class String.

2) In StringBuilder and StringBuffer, there is a
    method append(StringBuffer sb) but no
    append(StringBuilder).

In case 2, the the code :
   StringBuilder b=new StringBuilder.append(new StringBuilder());

I think the method append(CharSequence) must be called instead.
It works but i think that a method append(StringBuilder) could
perform better using getChar(...) instead of
a loop over each character.

The append(StringBuilder) could be inserted to
AbstractStringBuilder.

Moreover, the class par of the String documentation have
lot of links to StringBuffer. I think, all this links
could be transfered to StringBuilder.

PS: lot of methods in AbstractStringBuilder return an
   AbstractStringBuilder that is not used in overriden
   counterparts in StringBuffer and StringBuilder.
   Perhaps, this methods could return void to avoid
   a supplementary pop opcode (poor man optimisation :)