ArgType for String should probably not call ToString on all types

[mitsuhiko]

There are some types which probably are fine to stringify implicity:

• bool
• number
• string

But there are some where you can get really confusing behavior. The most notable one is Kwargs. This for instance can cause some unknown keyword arguments to be accepted and converted (see #594).

{{ ('hello', 'world') | join(d=' ') }}
  --> hello{"d": " "}world

[mitsuhiko]

On further thought we actually only want to disallow Kwargs here somehow. The join filter actually does stringify anything on Jinja2.

[mitsuhiko]

I think the real issue is that we're using a Value to funnel keyword arguments and that is just always naughty. At the very least the ArgType implementation for the string types should refuse to stringify a kwargs object but I feel like most filters and functions would not want to magically have a Kwargs value show up in Value either.

So one option would be to say that ArgType for Value actually refuses to accept a Kwargs object and a separate ValueOrKwargs type would be needed to hold it. That would break some stuff but probably make it way more explicit about what is happening.

The part of Kwargs not stringifying I think should be fixed as a bugfix because that for sure was unintentional.