docs: more specific description of strftime

docs/source/filters/date.md

@@ -2,7 +2,7 @@
 title: date
 ---
 
-Converts a timestamp into another date format. The format for this syntax is the same as [`strftime`](http://strftime.net). The input is first converted to Date object by [new Date()][newDate].
+Converts a timestamp into another date format. LiquidJS tries to be conformant with Shopify/Liquid which is using Ruby's core [Time#strftime(string)](http://www.ruby-doc.org/core/Time.html#method-i-strftime). The input is firstly converted to `Date` object via [new Date()][newDate].
 
 Input
 ```liquid

docs/source/zh-cn/filters/date.md

@@ -2,7 +2,7 @@
 title: date
 ---
 
-把时间戳转换为字符串，这一语法的格式同 [`strftime`](http://strftime.net)。LiquidJS 会先通过 [new Date()][newDate] 尝试把输入转换为 Date 对象。
+把时间戳转换为字符串。LiquidJS 尝试跟 Shopify/Liquid 保持一致，它用的是 Ruby 核心的 [Time#strftime(string)](http://www.ruby-doc.org/core/Time.html#method-i-strftime)。此外 LiquidJS 会先通过 [new Date()][newDate] 尝试把输入转换为 Date 对象。
 
 输入
 ```liquid

test/integration/builtin/tags/assign.ts

@@ -75,11 +75,17 @@ describe('tags/assign', function () {
       const html = await liquid.parseAndRender(src, { num: 1 })
       return expect(html).to.equal('11')
     })
-    it('should write to the belonging scope', async function () {
+    it('should write to the root scope', async function () {
       const src = '{%for a in (1..2)%}{%assign num = a%}{{a}}{%endfor%} {{num}}'
       const html = await liquid.parseAndRender(src, { num: 1 })
       return expect(html).to.equal('12 2')
     })
+    it('should not change input scope', async function () {
+      const src = '{%for a in (1..2)%}{%assign num = a%}{{a}}{%endfor%} {{num}}'
+      const ctx = { num: 1 }
+      await liquid.parseAndRender(src, ctx)
+      return expect(ctx.num).to.equal(1)
+    })
   })
   it('should support sync', function () {
     const src = '{% assign foo="bar" %}{{foo}}'