fix incorrect error about bare cr/lf when sending a message over smtp

we weren't properly tracking the cr's and lf's when being strict about message
lines when sending data.

we are reading buffered data from a Reader. if that chunk happens to start with
a newline, we weren't looking at the previously written data, which could be a
cr. instead, in that case, we would always claim the cr/lf wasn't correct.

the new test case triggered the behaviour before having the fix.

should solve issue #129 by x8x, thanks for the report!

smtp/data.go

@@ -40,8 +40,10 @@ func DataWrite(w io.Writer, r io.Reader) error {
 					c := p[n]
 					if c == '\n' {
 						if firstcr < 0 {
-							// Bare newline.
-							return ErrCRLF
+							if n > 0 || last != '\r' {
+								// Bare newline.
+								return ErrCRLF
+							}
 						} else if firstcr != n-1 {
 							// Bare carriage return.
 							return ErrCRLF