goprotobuf: Use p.isUnmarshaler consistently in decode.go.

Fixes #41.

R=crawshaw, r
CC=golang-dev
https://codereview.appspot.com/10474043
diff --git a/proto/decode.go b/proto/decode.go
index e0e9445..a38d391 100644
--- a/proto/decode.go
+++ b/proto/decode.go
@@ -645,7 +645,7 @@
 	}
 
 	// If the object can unmarshal itself, let it.
-	if p.isMarshaler {
+	if p.isUnmarshaler {
 		iv := structPointer_Interface(bas, p.stype)
 		return iv.(Unmarshaler).Unmarshal(raw)
 	}