Allow to return list as JSON

[greyli]

This adds support to serializing list return values to JSON, like what we did for the dictionary:

@app.route('/')
def index():
    return ['this', 'will', 'become', 'JSON']

There are some discusses related to this feature in #3111, and the concern is the list return value looks too much like the tuple. Maybe we can rethink about this feature? I propose this because the following reasons:

• List as JSON response is a common use case for Web APIs. We support returning dict as JSON, but not list. This makes people feel inconsistent (Could list responses also be converted to JSON automatically? #4533, Flask return a list directly #4659).
• With fix #2736 - remove explicit check for list as a return value in make_response #2737, we only treat tuple as a valid shortcut for return values, so there will be no program conflicts to allow returning list (we also throw an error for list return value currently).
• In terms of semantics, there is no concept of "tuple" in JSON. Although list and tuple are both sequences in Python, there are differences in syntax and meaning. We can add docs to address the different meaning of list and tuple in return values.
• In terms of visuals, most people will not mix [1, 2, 3] and (1, 2, 3), and people usually return tuple items without the bracket in the view function.

If this change is acceptable, I will update the docs and changelog.

Sorry for break the "zero status". :P

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.

[greyli]

The failing "Pallets Development Versions" check is due to a missing import in Werkzueg (werkzeug.routing.RoutingException), I will make a PR to fix it later.

[davidism]

Looks like that fix should be made in Werkzeug @pgjones

[davidism]

I think my original hesitation was that returning a dict is explicitly different from returning a database model. Returning a list of database results is probably something a lot more users will try. But I guess it's not that different to say they have to do obj.to_json() and also support [obj.to_json() for obj in objs].

[greyli]

I see. If they don't return a list of objects directly and call jsonify for the list, they will get the same error. Maybe we could update the docs to mention that the ORM/ODM object is not jsonify-able, they need to convert them to a dict or use a serialization library.

[davidism]

Another potential confusion now that we support returning iterators/generators is that you're going to get very different results doing return ["a", "b", "c"] or return iter(["a", "b", "c"]).