Prevent use of extended query protocol for BEGIN before COPY (#1639)

When autocommit is off and the first query in a transaction is a COPY, we were
sending a BEGIN using the extended query protocol. It was being permitted to
use named portals as well, ignoring prepareThreshold.

Fix by forcing simple query mode and marking the BEGIN as oneshot.

Fixes #1638

pgjdbc/src/main/java/org/postgresql/core/v3/QueryExecutorImpl.java

@@ -630,8 +630,12 @@ public void handleWarning(SQLWarning warning) {
       };
 
       try {
-        sendOneQuery(beginTransactionQuery, SimpleQuery.NO_PARAMETERS, 0, 0,
-            QueryExecutor.QUERY_NO_METADATA);
+        /* Send BEGIN with simple protocol preferred */
+        int beginFlags = QueryExecutor.QUERY_NO_METADATA
+                         | QueryExecutor.QUERY_ONESHOT
+                         | QueryExecutor.QUERY_EXECUTE_AS_SIMPLE;
+        beginFlags = updateQueryMode(beginFlags);
+        sendOneQuery(beginTransactionQuery, SimpleQuery.NO_PARAMETERS, 0, 0, beginFlags);
         sendSync();
         processResults(handler, 0);
         estimatedReceiveBufferBytes = 0;