Make JSON prettifying optional

[Kijewski]

This PR adds an optional argument to the |tojson filter, which controls if the serialized JSON data gets prettified or not. The arguments works the same as flask's |tojson filter, which passes the argument to python's json.dumps():

• Omitting the argument, providing a negative integer, or None, then compact JSON data is generated.
• Providing a non-negative integer, then this amount of ASCII spaces is used to indent the data. (Capped to 16 characters.)
• Providing a string, then this string is used as prefix. No attempts are made to ensure that the prefix actually consists of whitespaces, because chances are, that if you provide e.g. &nsbp;, then you are doing it intentionally.

This is a breaking change, because it changes the default behavior to not prettify the data. This is done intentionally, because this is how it works in flask.

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This PR is based on #1008, i.e. only the last commit is actually part of this PR.

Resolves #1019.

[Kijewski]

Are there open questions left?

[GuillaumeGomez]

I'm not a big fan of this change. Wouldn't it better to have different filters for each case? Or have the current json filter unchanged and new one which allows to pass options?

[djc]

I'm not a big fan of this change. Wouldn't it better to have different filters for each case? Or have the current json filter unchanged and new one which allows to pass options?

Why aren't you a fan?

[GuillaumeGomez]

Mostly because most of the time, the current json filter does exactly what I want. Rust doesn't allow to have default values if you don't specify them, which is fine but not really compatible with jinja here. So in case you don't want the default filter values, you could call the other json filter (json_with_options for example) and give the options you want for this call.

[djc]

Mostly because most of the time, the current json filter does exactly what I want. Rust doesn't allow to have default values if you don't specify them, which is fine but not really compatible with jinja here.

So you want prettified JSON most of the time? While Rust doesn't allow default values, the Askama template language can.

[GuillaumeGomez]

If you mean allowing a variable number of arguments to the json filter, then yes, I very much like this approach.