Remove tojson filter

[greyli]

Jinja2 added the tojson filter in version 2.9. Now that we bumped the dependency's versions to the latest, maybe we should delete it in Flask.

[ThiefMaster]

I somehow doubt that this logic still runs after your change: https://github.com/greyli/flask/blob/master/flask/json/__init__.py#L84-L100

[greyli]

@ThiefMaster It seems that the function you linked is used by jsonify(). Do you mean we should remove htmlsafe_dump() and htmlsafe_dumps()?

[ThiefMaster]

No, what I mean is that your PR changes the behavior:

• tojson_filter calls htmlsafe_dumps
• htmlsafe_dumps calls htmlsafe_dumps
• htmlsafe_dumps calls dumps
• dumps calls _dump_arg_defaults, which is the function I mentioned in my last comment.

But the Jinja version of the filter does not do this! I see two options here:

• Close this PR and leave it as-is - i.e. let flask replace the builtin Jinja filter as is the case right now, which works fine
• Populate the json.dumps_function and json.dumps_kwargs policies of Jinja when setting up the Jinja environment in Flask.

But since _dump_arg_defaults depends on application state (e.g. the blueprint handling the current request) the second option is not really feasible without very nasty hacks, so I think we'll just keep the current implementation.