[SabreAMF] String references serializing AMF3

[Jim Mischel]

SabreAMF currently does not cache strings and create references when 
serializing AMF3.  The writeString() function in 
SabreAMF/AMF3/Serializer.php always writes the literal string.  It 
doesn't look terribly difficult to cache the strings so that 
writeString() can create references if needed.  Has anybody done this?  
If so, I'd like to see the code.  If not, I'll code it up.

Jim