encoding/xml: fix spurious "no semicolon" in error

Noticed while doing other XML investigations.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/7550045

src/pkg/encoding/xml/xml.go

@@ -956,7 +956,7 @@ Input:
 				b0, b1 = 0, 0
 				continue Input
 			}
-			ent := string(d.buf.Bytes()[before])
+			ent := string(d.buf.Bytes()[before:])
 			if ent[len(ent)-1] != ';' {
 				ent += " (no semicolon)"
 			}

src/pkg/encoding/xml/xml_test.go

@@ -595,13 +595,6 @@ func TestEntityInsideCDATA(t *testing.T) {
 	}
 }
 
-// The last three tests (respectively one for characters in attribute
-// names and two for character entities) pass not because of code
-// changed for issue 1259, but instead pass with the given messages
-// from other parts of xml.Decoder.  I provide these to note the
-// current behavior of situations where one might think that character
-// range checking would detect the error, but it does not in fact.
-
 var characterTests = []struct {
 	in  string
 	err string
@@ -611,8 +604,10 @@ var characterTests = []struct {
 	{"\xef\xbf\xbe<doc/>", "illegal character code U+FFFE"},
 	{"<?xml version=\"1.0\"?><doc>\r\n<hiya/>\x07<toots/></doc>", "illegal character code U+0007"},
 	{"<?xml version=\"1.0\"?><doc \x12='value'>what's up</doc>", "expected attribute name in element"},
+	{"<doc>&abc\x01;</doc>", "invalid character entity &abc (no semicolon)"},
 	{"<doc>&\x01;</doc>", "invalid character entity & (no semicolon)"},
-	{"<doc>&\xef\xbf\xbe;</doc>", "invalid character entity & (no semicolon)"},
+	{"<doc>&\xef\xbf\xbe;</doc>", "invalid character entity &\uFFFE;"},
+	{"<doc>&hello;</doc>", "invalid character entity &hello;"},
 }
 
 func TestDisallowedCharacters(t *testing.T) {
@@ -629,7 +624,7 @@ func TestDisallowedCharacters(t *testing.T) {
 			t.Fatalf("input %d d.Token() = _, %v, want _, *SyntaxError", i, err)
 		}
 		if synerr.Msg != tt.err {
-			t.Fatalf("input %d synerr.Msg wrong: want '%s', got '%s'", i, tt.err, synerr.Msg)
+			t.Fatalf("input %d synerr.Msg wrong: want %q, got %q", i, tt.err, synerr.Msg)
 		}
 	}
 }