Fix inconsistent calling of template functions in PutN in stream.h. When
used with a GenericStringBuffer<<UTF8>, MemoryPoolAllocator>, PutN would call
PutReserve from stream.h, and PutUnsafe from stringbuffer.h. This
resulted in bytes being added to the buffer without allocating space.
This was not an issue when used with the default memory allocator,
because in this case the specialized PutN is used from stringbuffer.h.
The tests state the current traits of types Document, Value and
StringBuffer. There are slight differences between them. It seems like a
good idea to extend this approach across more types, and to review the
expected traits across the board.
