fix: enumerate Promises (e.g. in for & tablerow) (#237)

* fix: enumerate Promise<array> (e.g. in {% for ... %})

Previously, a Promise of an array was not being enumerated in
{% for %}, for example. This is misleading since the library
handles promises elsewhere (e.g. if you {% assign x = promiseArray %}
and then {% for v in x %}, it worked just fine.

This PR makes Promises of arrays handled by changing toEnumerable
to handle then-ables. This affects other iterators, too, e.g. tablerow,
so I put in a test for that as well.

test/integration/builtin/tags/for.ts

@@ -20,15 +20,23 @@ describe('tags/for', function () {
       nullProtoObj: Object.create(null),
       obj: { foo: 'bar', coo: 'haa' },
       alpha: ['a', 'b', 'c'],
+      promiseArray: Promise.resolve(['a', 'b', 'c']),
       emptyArray: []
     }
   })
+
   it('should support array', async function () {
     const src = '{%for c in alpha%}{{c}}{%endfor%}'
     const html = await liquid.parseAndRender(src, scope)
     return expect(html).to.equal('abc')
   })
 
+  it('should support promise of array', async function () {
+    const src = '{%for c in promiseArray%}{{c}}{%endfor%}'
+    const html = await liquid.parseAndRender(src, scope)
+    return expect(html).to.equal('abc')
+  })
+
   it('should support object', async function () {
     const src = '{%for item in obj%}{{item[0]}},{{item[1]}}-{%else%}b{%endfor%}'
     const html = await liquid.parseAndRender(src, scope)

test/integration/builtin/tags/tablerow.ts

@@ -14,6 +14,16 @@ describe('tags/tablerow', function () {
     return expect(html).to.equal(dst)
   })
 
+  it('should support promises', async function () {
+    const src = '{% tablerow i in promiseNumbers %}{{ i }}{% endtablerow %}'
+    const ctx = {
+      promiseNumbers: Promise.resolve([1,2,3])
+    }
+    const dst = '<tr class="row1"><td class="col1">1</td><td class="col2">2</td><td class="col3">3</td></tr>'
+    const html = await liquid.parseAndRender(src, ctx)
+    return expect(html).to.equal(dst)
+  })
+
   it('should support cols', async function () {
     const src = '{% tablerow i in alpha cols:2 %}{{ i }}{% endtablerow %}'
     const ctx = {