Loose end: collect signature

[Brian Goetz]

In reviewing the docs, I notice that the signature for the three-arg 
collect() is gratuitously out of sync with Collector.

Collector is a tuple of ( Supplier<A>, BiConsumer<A,T>, 
BinaryOperator<A> ).  But collect() takes a Supplier<A>, a 
BiConsumer<A,T>, and a BiConsumer<A,A>.

I think we should change the three-arg collect() to match Collector.