Fix #1748 where allowed prototype methods are not called

[aalimovs]

Fixes #1748 where allowed prototype methods are not called.

Without this fix the official documentation is incorrect:

const template = handlebars.compile("{{aString.trim}}");
const result = template(
  { aString: "  abc  " },
  {
    allowedProtoMethods: {
      trim: true
    }
  }
);
// result = 'abc'

The result is [object Object], not abc.

[jaylinski]

Note: failing test was not caused by this PR and was fixed in #1963. Needs rebase.

[aalimovs]

@jaylinski rebased, but I've no idea why this PR now picks up changes in .github/workflows/ci.yml that are already in 4.x branch. I've tried rebasing the fork few different ways but still the same.

If you want me to recreate the PR let me know.

[jaylinski]

No problem, I fixed it.

[jaylinski]

Why did you use parent[propertyName]() instead of result()? Any specific reason?

[aalimovs]

It doesn't work with result(), complains that:

TypeError: String.prototype.trim called on null or undefined

I'm not 100% sure why, but as I understand result loses this context.