hstore.handling.mode docs seem inaccurate (and map shows null values)

There are 2 issues:

1. Docs are inaccurate regarding default

   hstore.handling.mode

1. `map` loses data in avro consumer

The docs say that the default mode for `hstore.handling.mode` is `map`, however, I am finding that it is actually `json`:

https://debezium.io/documentation/reference/1.0/connectors/postgresql.html#hstore-values
HStore Values

When hstore.handling.mode configuration property is set to map, then the connector will use the java.util.Map<String,String> logical type, MAP schema type for all HSTORE columns. This is the default mode.

We need to either fix the docs to say `json` is the default, or actually make `map` the default.

I also am finding that using kafka-avro-console-consumer, with `hstore` fields using `map` as the `hstore.handling.mode`, it just shows as null:
pg:
```

1. SELECT here_some_hstore FROM table;
   here_some_hstore
   --------------------
   "a"=>"1", "b"=>"2"
   ```

kafka-avro-console-consumer
```
"here_some_hstore":null
```