map: fix in for deleted keys

vlib/builtin/map.v

@@ -16,7 +16,7 @@ pub:
 struct mapnode {
 	left &mapnode
 	right &mapnode
-	is_empty bool
+	is_empty bool // set by delete()
 	key string
 	val voidptr
 }
@@ -97,7 +97,7 @@ fn (n & mapnode) find(key string, out voidptr, element_size int) bool{
 
 // same as `find`, but doesn't return a value. Used by `exists`
 fn (n & mapnode) find2(key string, element_size int) bool{
-	if n.key == key {
+	if n.key == key && !n.is_empty {
 		return true
 	}
 	else if n.key > key {

vlib/builtin/map_test.v

@@ -168,6 +168,17 @@ fn test_mut_arg() {
 	assert m['a'] == 10
 }
 
+fn test_delete() {
+	mut m := map[string]int
+	m['one'] = 1
+	m['two'] = 2
+	println(m['two']) // => "2"
+	m.delete('two')
+	println(m['two']) // => 0
+	assert 'two' in m == false
+	println('two' in m) // => true, on Linux  and Windows  <-- wrong !
+}	
+
 /*
 fn test_ref() {
 	m := { 'one': 1 }