Fixed incorrect String.length (#238)

* Fixed incorrect String.length

pkg/runtime/values/string.go

@@ -106,7 +106,7 @@ func (t String) Copy() core.Value {
 }
 
 func (t String) Length() Int {
-	return Int(len(t))
+	return Int(len([]rune(string(t))))
 }
 
 func (t String) Contains(other String) Boolean {

pkg/runtime/values/string_test.go

@@ -26,4 +26,12 @@ func TestString(t *testing.T) {
 			So(v.Hash(), ShouldEqual, v.Hash())
 		})
 	})
+
+	Convey(".Length", t, func() {
+		Convey("Should return unicode length", func() {
+			str := values.NewString("Спутник")
+
+			So(str.Length(), ShouldEqual, 7)
+		})
+	})
 }