fix #3217 (string.replace)

vlib/builtin/string.v

@@ -152,7 +152,7 @@ pub fn (s string) replace(rep, with string) string {
 			break
 		}
 		idxs << idx
-		idx++
+		idx += rep.len
 	}
 	// Dont change the string if there's nothing to replace
 	if idxs.len == 0 {

vlib/builtin/string_test.v

@@ -220,6 +220,9 @@ fn test_replace() {
 	assert b.replace('charptr', 'byteptr') == '*byteptr'
 	c :='abc'
 	assert c.replace('','-') == c
+	v :='a   b c d'
+	assert v.replace('  ',' ') == 'a  b c d'
+
 }
 
 fn test_replace_each() {