bytes, strings: improve Title test coverage by adding cases with underscore and unicode line separator

R=golang-codereviews, gobot, r
CC=golang-codereviews
https://golang.org/cl/42310045

src/pkg/bytes/bytes_test.go

@@ -1073,6 +1073,8 @@ var TitleTests = []TitleTest{
 	{"123a456", "123a456"},
 	{"double-blind", "Double-Blind"},
 	{"ÿøû", "Ÿøû"},
+	{"with_underscore", "With_underscore"},
+	{"unicode \xe2\x80\xa8 line separator", "Unicode \xe2\x80\xa8 Line Separator"},
 }
 
 func TestTitle(t *testing.T) {