mime: Fix BenchmarkQDecodeHeader to call DecodeHeader

Found this while reading through the code. The benchmark
accidently called the wrong function.

Change-Id: Idb88aa71e7098a4e29e7f5f39e64f8c5f8936a2a
Reviewed-on: https://go-review.googlesource.com/19977Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/mime/encodedword_test.go

@@ -203,6 +203,6 @@ func BenchmarkQDecodeHeader(b *testing.B) {
 	dec := new(WordDecoder)
 
 	for i := 0; i < b.N; i++ {
-		dec.Decode("=?utf-8?q?=C2=A1Hola,_se=C3=B1or!?=")
+		dec.DecodeHeader("=?utf-8?q?=C2=A1Hola,_se=C3=B1or!?=")
 	}
 }