refactor: make PgStream implements Flushable (#1008)

pgjdbc/src/main/java/org/postgresql/core/PGStream.java

@@ -14,6 +14,7 @@
import java.io.Closeable;
import java.io.EOFException;
import java.io.FilterOutputStream;
import java.io.Flushable;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
@@ -30,7 +31,7 @@
 * In general, instances of PGStream are not threadsafe; the caller must ensure that only one thread
 * at a time is accessing a particular PGStream instance.
 */
public class PGStream implements Closeable {
public class PGStream implements Closeable, Flushable {
  private final SocketFactory socketFactory;
  private final HostSpec hostSpec;

@@ -502,6 +503,7 @@ public void sendStream(InputStream inStream, int remaining) throws IOException {
   *
   * @throws IOException if an I/O error occurs
   */
  @Override
  public void flush() throws IOException {
    if (encodingWriter != null) {
      encodingWriter.flush();