Fixes for BufferedWriter and LineBufferedWriter

BufferedWriter::inner flushes before returning the underlying writer.

BufferedWriter::write no longer flushes the underlying writer.

LineBufferedWriter::write flushes up to the *last* newline in the input
string, not the first.
auto merge of #10738 : sfackler/rust/buffered-fixes, r=alexcrichton

BufferedWriter::inner flushes before returning the underlying writer.

BufferedWriter::write no longer flushes the underlying writer.

LineBufferedWriter::write flushes up to the *last* newline in the input
string, not the first.