[bug] Lambda functions return a function, not a value

[Potherca]

When comparing the test for tiny-mustache against the results of mustache.js, I noticed there is a bug in the current lambda implementation.

The Mustache manual states:

Lambdas

When the value is a callable object, such as a function or lambda, the object will be invoked and passed the block of text. The text passed is the literal block, unrendered. {{tags}} will not have been expanded - the lambda should do that on its own. In this way you can implement filters or caching.

Template:

{{#wrapped}}
  {{name}} is awesome.
{{/wrapped}}

Hash:

{
  "name": "Willy",
  "wrapped": function() {
    return function(text, render) {
      return "<b>" + render(text) + "</b>"
    }
  }
}

Output:

<b>Willy is awesome.</b>

However, instead of a function that returns a function, tiny-mustache expects a plain function (i.e. a function that returns a scalar).

[Potherca]

A naive fix would be to change

https://github.com/aishikaty/tiny-mustache/blob/ec53eedf187114996a0ef984a31ddf9b4cf19d10/mustache.js#L45

to

              output += name().call(ctx, childCode, function (template) {

but that would break backward compatibility. A check could be added, but I am not sure how that matches the desire to keep the code as small as possible.