PooledConnection.close violates io.r2dbc.spi.Closeable contract by not being idempotent

[SanjayVas]

Bug Report

Versions

N/A: Library issue at HEAD and in latest release (v1.0.2.RELEASE)

Current Behavior

Calling close() on a PooledConnection object that is already closed throws an IllegalStateException.

This method is overridden from the io.r2dbc.spi.Closeable interface, which states

If the object is already closed, then {@link Publisher#subscribe(Subscriber) subscriptions} complete successfully and the close operation has no effect.

Stack trace

java.lang.IllegalStateException: The connection is closed
    at io.r2dbc.pool.PooledConnection.assertNotClosed(PooledConnection.java:208)
    at io.r2dbc.pool.PooledConnection.close(PooledConnection.java:110)
    at io.r2dbc.pool.PooledConnection.close(PooledConnection.java:44)

Steps to reproduce

val connection: Connection = getPooledConnection()
connection.close().awaitFirstOrNull()
connection.close()  // Throws IllegalStateException

Expected behavior/code

No exception is thrown by close() if the Connection is already closed.
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