[R] R class accessor problem in 2.6 - FIXED

[Joseph Wang]

The reason R-swig was not working on 2.6 was that the C code was not setting 
the S4 flag.  On 2.5, R would still treat it as an S4 class, but in 2.6, it 
is necessary to have this flag set for it to be treated as S4.

I've checked in the fix to swig-trunk.  There were no C coding changes, just a 
new r.swg file.  The warning message for 2.6 has also been removed.