fix_replace

builtin/string.v

@@ -75,7 +75,7 @@ pub fn (s string) cstr() byteptr {
 
 pub fn (s string) replace(rep, with string) string {
 	if s.len == 0 || rep.len == 0 {
-		return ''
+		return s
 	}
 	if !s.contains(rep) {
 		return s

builtin/string_test.v

@@ -165,6 +165,8 @@ fn test_replace() {
 	assert b.replace('B', '') == 'onetwothree'
 	b = '**char'
 	assert b.replace('*char', 'byteptr') == '*byteptr'
+	mut c :='abc'
+	assert c.replace('','-') == c
 }
 
 fn test_itoa() {