pgconn.CommandTag is now an opaque type

It now makes a copy instead of retaining driver memory. This is in
preparation to reuse the driver read buffer.

query_test.go

@@ -45,7 +45,7 @@ func TestConnQueryScan(t *testing.T) {
 		t.Fatalf("conn.Query failed: %v", err)
 	}
 
-	assert.Equal(t, "SELECT 10", string(rows.CommandTag()))
+	assert.Equal(t, "SELECT 10", rows.CommandTag().String())
 
 	if rowCount != 10 {
 		t.Error("Select called onDataRow wrong number of times")
@@ -79,7 +79,7 @@ func TestConnQueryWithoutResultSetCommandTag(t *testing.T) {
 	assert.NoError(t, err)
 	rows.Close()
 	assert.NoError(t, rows.Err())
-	assert.Equal(t, "CREATE TABLE", string(rows.CommandTag()))
+	assert.Equal(t, "CREATE TABLE", rows.CommandTag().String())
 }
 
 func TestConnQueryScanWithManyColumns(t *testing.T) {
@@ -1139,7 +1139,7 @@ func TestConnQueryDatabaseSQLDriverValuerWithAutoGeneratedPointerReceiver(t *tes
 	if err != nil {
 		t.Fatal(err)
 	}
-	if string(commandTag) != "INSERT 0 1" {
+	if commandTag.String() != "INSERT 0 1" {
 		t.Fatalf("want %s, got %s", "INSERT 0 1", commandTag)
 	}
 
@@ -1976,7 +1976,7 @@ func TestConnQueryFuncAbort(t *testing.T) {
 			},
 		)
 		require.EqualError(t, err, "abort")
-		require.Nil(t, ct)
+		require.Equal(t, pgconn.CommandTag{}, ct)
 	})
 }