html: add tests for UnescapeString edge cases

R=golang-dev, gobot, bradfitz
CC=golang-dev
https://golang.org/cl/40810044

src/pkg/html/escape_test.go

@@ -64,6 +64,24 @@ var unescapeTests = []unescapeTest{
 		"Footnote‡",
 		"Footnote‡",
 	},
+	// Handle single ampersand.
+	{
+		"copySingleAmpersand",
+		"&",
+		"&",
+	},
+	// Handle ampersand followed by non-entity.
+	{
+		"copyAmpersandNonEntity",
+		"text &test",
+		"text &test",
+	},
+	// Handle "&#".
+	{
+		"copyAmpersandHash",
+		"text &#",
+		"text &#",
+	},
 }
 
 func TestUnescape(t *testing.T) {