Fix array-like handling in JSON.stringify

This fixes robertkrimen#73

bug_test.go

@@ -313,6 +313,19 @@ func Test_issue64(t *testing.T) {
 	})
 }
 
+func Test_issue73(t *testing.T) {
+	tt(t, func() {
+		test, vm := test()
+
+		vm.Set("abc", [4]int{3, 2, 1, 0})
+
+		test(`
+            var def = [ 0, 1, 2, 3 ];
+            JSON.stringify(def) + JSON.stringify(abc);
+        `, "[0,1,2,3][3,2,1,0]")
+	})
+}
+
 func Test_7_3_1(t *testing.T) {
 	tt(t, func() {
 		test(`

builtin_json.go

@@ -3,6 +3,7 @@ package otto
 import (
 	"bytes"
 	"encoding/json"
+	"fmt"
 	"math"
 	"strings"
 )
@@ -250,7 +251,17 @@ func builtinJSON_stringifyWalk(ctx _builtinJSON_stringifyContext, key string, ho
 			defer func() { ctx.stack = ctx.stack[:len(ctx.stack)-1] }()
 		}
 		if isArray(holder) {
-			length := holder.get("length").value.(uint32)
+			var length uint32
+			switch value := holder.get("length").value.(type) {
+			case uint32:
+				length = value
+			case int:
+				if value >= 0 {
+					length = uint32(value)
+				}
+			default:
+				panic(newTypeError(fmt.Sprintf("JSON.stringify: invalid length: %v (%[1]T)", value)))
+			}
 			array := make([]interface{}, length)
 			for index, _ := range array {
 				name := arrayIndexToString(int64(index))