json(b)_to_tsvector with numeric values

Hi,

We've just noticed, that current implementation of `json(b)_to_tsvector` can be
confusing sometimes, if the target document contains numeric values.
In this case
we just drop them, and only string values will contribute to the result:

select to_tsvector('english', '{"a": "The Fat Rats", "b": 123}'::jsonb);
to_tsvector
-----------------
'fat':2 'rat':3
(1 row)

The result would be less surprising if all values, that can be converted to
string representation (so, strings and numeric values, nothing to do for null &
boolean), will take part in it:

select to_tsvector('english', '{"a": "The Fat Rats", "b": 123}'::jsonb);
to_tsvector
-------------------------
'123':5 'fat':2 'rat':3
(1 row)

Attached patch contains small fix that's necessary to get the described
behavior. This patch doesn't touch `ts_headline` though, because following the
same approach it would require changing the type of element in the resulting
json(b).

Any opinions about this suggestion? Can it be considered as a bug fix and
included into this release?