Introduce fast-path handling for decoding primitive values

[mp911de]

Right now, PostgresqlRow.get(…) leads to an exception when trying to decode primitive values regardless of whether the value is present or absent. It would be good to handle primitive value decoding in a lenient way and only throw an exception if the value is absent and therefore, cannot be represented.

java.lang.IllegalArgumentException: Cannot decode value of type int with OID 23
        at io.r2dbc.postgresql.codec.DefaultCodecs.decode(DefaultCodecs.java:236)
        at io.r2dbc.postgresql.PostgresqlRow.decode(PostgresqlRow.java:115)
        at io.r2dbc.postgresql.PostgresqlRow.get(PostgresqlRow.java:86)
        at org.springframework.r2dbc.core.BeanPropertyRowMapper.getItemValue(BeanPropertyRowMapper.java:241)