encoding/xml: use bytes.Buffer.WriteString

Change-Id: Icb0b917c48252ed8831e6c07109c1bc2d2438a42
Reviewed-on: https://go-review.googlesource.com/c/go/+/421234
Reviewed-by: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: hopehook <[email protected]>
Run-TryBot: Daniel Martí <[email protected]>
Reviewed-by: Dan Kortschak <[email protected]>

src/encoding/xml/xml.go

@@ -1100,7 +1100,7 @@ Input:
 
 			if haveText {
 				d.buf.Truncate(before)
-				d.buf.Write([]byte(text))
+				d.buf.WriteString(text)
 				b0, b1 = 0, 0
 				continue Input
 			}