Fix join() filter with d='val' syntax to be parity with Python Jinja2

[tjsmith-meta]

Description / Reproduction steps

The following template behaves differently with MiniJinja 2.3.1 vs Python Jinja2 2.11.3.

{{ ('hello', 'world') | join(d=' ') }}

MiniJinja output: hello{"d": " "}world
Python Jinja2 output: hello world

If MiniJinja intends not to support this syntax, I would expect that it might make sense for it to fail / return error instead of producing an output not consistent with Python Jinja2.

There's an example in the Python Jinja2 documentation that seems to suggest this syntax is part of the spec.
https://jinja.palletsprojects.com/en/2.11.x/templates/#join

{{ users|join(', ', attribute='username') }}

In the example, the attribute argument to join uses that sort of syntax, so it seems to suggest that the syntax should work for d as well.

[mitsuhiko]

Filters do not have perfect parity. The attribute part definitely makes sense supporting, the d part is a bit weird. A workaround for you today is the following:

{{ users|map(attribute='username')|join(', ') }}