Support JSON data type in `insert_rows` and as query parameters

[tswast]

We need to make sure the JSON data type is supported for insert_rows. I suspect it is broken for query parameters, too.

See: #1587 (comment)

[tswast]

I think we need to update:

python-bigquery/google/cloud/bigquery/_helpers.py

Line 442 in fdd580f

"JSON": _json_from_json,

It should be json_to_json and dumps rather than loads.

[tswast]

Also, update our insert_rows and query_parameters system tests to use the JSON data type in some column.

[ekampf]

@tswast _json_from_json expects value and field but the code calling it in _scalar_field_to_json calls converter(value) which would always fail...

[tswast]

@ekampf You are correct. I fix this in #1757