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.

src/builtin/tags/for.ts

@@ -36,7 +36,7 @@ export default {
   },
   render: function * (ctx: Context, emitter: Emitter) {
     const r = this.liquid.renderer
-    let collection = toEnumerable(evalToken(this.collection, ctx))
+    let collection = toEnumerable(yield evalToken(this.collection, ctx))
 
     if (!collection.length) {
       yield r.renderTemplates(this.elseTemplates, ctx, emitter)

src/builtin/tags/tablerow.ts

@@ -30,7 +30,7 @@ export default {
   },
 
   render: function * (ctx: Context, emitter: Emitter) {
-    let collection = toEnumerable(evalToken(this.collection, ctx))
+    let collection = toEnumerable(yield evalToken(this.collection, ctx))
     const hash = yield this.hash.render(ctx)
     const offset = hash.offset || 0
     const limit = (hash.limit === undefined) ? collection.length : hash.limit