check JSON data transfer

in 10.5 Item::val_json was introduced (i.e. json getting method).

But it was not added to reference Items used in aggregation and VIEW handling.

Check that JSON stay JSON (not converted to string) as:
1) result of aggregate (? - not sure that it is meaningful)
2) field by which aggregated
3) parameter of aggregate function
4) view field

Here is example of turning json to string:

 SELECT JSON_ARRAY(_UTF8 'str', JSON_OBJECT(_LATIN1 'plugin', _LATIN1'unix_socket'));

 JSON_ARRAY(_UTF8 'str', JSON_OBJECT(_LATIN1 'plugin', _LATIN1'unix_socket'))

-["str", {"plugin": "unix_socket"}]

+["str", "{\"plugin\": \"unix_socket\"}"]

for each case there are 4cases:
1. constants:
1.1 with charset conversion
1.2 without
2. non-constants
2.1 with charset conversion
2.2 without