Allow ResultSet to update with PGobject

Postgres date/time support time zone and microsecond, but java.sql.Date/Time/Timestamp + Calendar can't support them well.

So when mapping java8 time to postgres date/time in [`slick-pg`](https://github.com/tminglei/slick-pg), I used `PGobject` as the delegate, instead of converting java8 time to java.sql.Date/Time/Timestamp then to postgres date/time.


It works well when I use `PreparedStatement` to insert/update records, but when I start to use [`slick`](https://github.com/slick/slick)'s `mutate`, which delegate to ResultSet's update, to update records, I encountered a ClassCastException as below:
```scala
[error] Test com.github.tminglei.slickpg.PgDate2SupportTest.testDatetimeFunctions failed: java.lang.ClassCastException: org.postgresql.util.PGobject cannot be cast to java.sql.Date
[error]     at org.postgresql.jdbc2.AbstractJdbc2ResultSet.updateRowBuffer(AbstractJdbc2ResultSet.java:1779)
[error]     at org.postgresql.jdbc2.AbstractJdbc2ResultSet.updateRow(AbstractJdbc2ResultSet.java:1391)
[error]     at scala.slick.jdbc.MutatingStatementInvoker$$anonfun$mutate$1$$anonfun$apply$mcV$sp$2$$anon$3.row_$eq(StatementInvoker.scala:101)
[error]     at com.github.tminglei.slickpg.PgDate2SupportTest$$anonfun$testDatetimeFunctions$1$$anonfun$72.apply(PgDate2SupportTest.scala:196)
[error]     at com.github.tminglei.slickpg.PgDate2SupportTest$$anonfun$testDatetimeFunctions$1$$anonfun$72.apply(PgDate2SupportTest.scala:195)
[error]     at scala.slick.jdbc.MutatingStatementInvoker$$anonfun$mutate$1$$anonfun$apply$mcV$sp$2.apply(StatementInvoker.scala:117)
[error]     at scala.slick.jdbc.MutatingStatementInvoker$$anonfun$mutate$1$$anonfun$apply$mcV$sp$2.apply(StatementInvoker.scala:92)
[error]     at scala.util.Either.fold(Either.scala:99)
[error]     at scala.slick.jdbc.MutatingStatementInvoker$$anonfun$mutate$1.apply$mcV$sp(StatementInvoker.scala:90)
[error]     at scala.slick.jdbc.MutatingStatementInvoker$$anonfun$mutate$1.apply(StatementInvoker.scala:90)
[error]     at scala.slick.jdbc.MutatingStatementInvoker$$anonfun$mutate$1.apply(StatementInvoker.scala:90)
[error]     at scala.slick.jdbc.JdbcBackend$BaseSession.withTransaction(JdbcBackend.scala:426)
[error]     at scala.slick.jdbc.MutatingStatementInvoker$class.mutate(StatementInvoker.scala:87)
[error]     at scala.slick.driver.JdbcInvokerComponent$QueryInvoker.mutate(JdbcInvokerComponent.scala:25)
[error]     at com.github.tminglei.slickpg.PgDate2SupportTest$$anonfun$testDatetimeFunctions$1.apply(PgDate2SupportTest.scala:195)
[error]     at com.github.tminglei.slickpg.PgDate2SupportTest$$anonfun$testDatetimeFunctions$1.apply(PgDate2SupportTest.scala:51)
[error]     at scala.slick.backend.DatabaseComponent$DatabaseDef$class.withSession(DatabaseComponent.scala:34)
[error]     at scala.slick.jdbc.JdbcBackend$DatabaseFactoryDef$$anon$4.withSession(JdbcBackend.scala:61)
[error]     at com.github.tminglei.slickpg.PgDate2SupportTest.testDatetimeFunctions(PgDate2SupportTest.scala:51)
[error]     ...
```

The cause is that `pgjdbc` try to cast the PGobject to java.sql.Date/Time/Timestamp when the type is DATE/TIME/TIMESTAMP.

So I sent this pull request to allow ResultSet to update with PGobject, which works as expected on my local.

Pls help review and give your suggestions.

Thanks,
Minglei Tu
NullObject is subclass of PGobject, and its 'getValue' method will return null.
So it's covered.