stream_wrap: use v8::Integer::NewFromUnsigned()

Use v8::Integer::NewFromUnsigned() when updating the writeQueueSize
field.

Before this commit, it used v8::Integer::New() but that takes an
int32_t. It's unlikely for a write queue to grow beyond 2**31-1 bytes
but let's use the unsigned integer constructor anyway, just in case.
multi-context wip