Make is_write_vectored return true for BufWriter
Make is_write_vectored return true for BufWriter

BufWriter provides an efficient implementation of
write_vectored also when the underlying writer does not
support vectored writes.

aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust