fix: broken Substitute

Substitution did not work if substitution map had more than one element.

fix #6

slug.go

@@ -84,7 +84,7 @@ func MakeLang(s string, lang string) (slug string) {
 func Substitute(s string, sub map[string]string) (buf string) {
 	buf = s
 	for key, val := range sub {
-		buf = strings.Replace(s, key, val, -1)
+		buf = strings.Replace(buf, key, val, -1)
 	}
 	return
 }

slug_test.go

@@ -92,6 +92,7 @@ func TestSlugMakeUserSubstituteLang(t *testing.T) {
 		{map[string]string{"'": " "}, "en", "That's great", "that-s-great"},
 		{map[string]string{"&": "or"}, "en", "This & that", "this-or-that"}, // by default "&" => "and"
 		{map[string]string{"&": "or"}, "de", "This & that", "this-or-that"}, // by default "&" => "und"
+		{map[string]string{"&": "or", "@": "the"}, "de", "@ This & that", "the-this-or-that"}, // by default "&" => "und", "@" => "an"
 	}
 
 	for index, smust := range testCases {
@@ -116,6 +117,8 @@ func TestSlugMakeSubstituteOrderLang(t *testing.T) {
 		want string
 	}{
 		{map[rune]string{'o': "left"}, map[string]string{"o": "right"}, "o o", "left-left"},
+		{map[rune]string{'o': "left", 'a': "r"}, map[string]string{"o": "right"}, "o a o", "left-r-left"},
+		{map[rune]string{'o': "left"}, map[string]string{"o": "right", "a": "r"}, "a o a o", "r-left-r-left"},
 		{map[rune]string{'&': "down"}, map[string]string{"&": "up"}, "&", "down"},
 	}
 
@@ -140,6 +143,7 @@ func TestSubstituteLang(t *testing.T) {
 		want string
 	}{
 		{map[string]string{"o": "no"}, "o o o", "no no no"},
+		{map[string]string{"o": "no", "a": "or"}, "o a o", "no or no"},
 		{map[string]string{"'": " "}, "That's great", "That s great"},
 	}
 
@@ -160,6 +164,7 @@ func TestSubstituteRuneLang(t *testing.T) {
 		want string
 	}{
 		{map[rune]string{'o': "no"}, "o o o", "no no no"},
+		{map[rune]string{'o': "no", 'a': "or"}, "o a o", "no or no"},
 		{map[rune]string{'\'': " "}, "That's great", "That s great"},
 	}