Description:
------------
dba_insert returns true if key already exists using handler 'flatfile'

Test script:
---------------
$path = __DIR__ . '/test.dba';
$mode = 'c';
$handler = 'flatfile';

@unlink($path);
$dba = dba_open($path, $mode, $handler);

// first insert success
var_dump(dba_insert('key', 'value', $dba));

// second insert failed -> already exists
var_dump(dba_insert('key', 'value', $dba));

Expected result:
----------------
bool(true)
PHP Warning:  dba_insert(key): Key already exists in /mnt/workspace/zf2/cache/tests/test_dba.php on line 15

Warning: dba_insert(key): Key already exists in /mnt/workspace/zf2/cache/tests/test_dba.php on line 15
bool(false)

Actual result:
--------------
bool(true)
PHP Warning:  dba_insert(key): Key already exists in /mnt/workspace/zf2/cache/tests/test_dba.php on line 15

Warning: dba_insert(key): Key already exists in /mnt/workspace/zf2/cache/tests/test_dba.php on line 15
bool(true)