Classlib: fixed PatternProxy constrainStream so defaultQuant works

Julian and I had a discussion about this on sc-users so I thought I'd turn it into a commit/push/pull.  Thanks Julian for the quick fix!  

// before the change:
Pdefn.defaultQuant = 4;
Pbindef(\x,\midinote,Pdefn(\m));
Pbindef(\x,\delta,0.5).play(quant:4);
Pdefn(\m,Pseq([60,62,64,65,67,65,64,62],inf)+12.rand);
// despite defaultQuant, every re-eval of line above takes effect immediately?