Improved performance of BlobType

I noticed that the `string` to `resource` conversion code in `BlobType` uses a quite inefficient technique.
This PR improves its performance with a simple change involving the `php://temp` stream.

Quick benchmark of the two approaches, converting a 10MB string:

    $value = str_repeat('x', 10 * 1024 * 1024);

    $t = microtime(true);
    $fp = fopen('data://text/plain;base64,' . base64_encode($value), 'r');
    printf("%.3fs\n", microtime(true) - $t);

    $t = microtime(true);
    $fp = fopen('php://temp', 'rb+');
    fwrite($fp, $value);
    fseek($fp, 0);
    printf("%.3fs\n", microtime(true) - $t);

Results on my laptop:

    0.090s
    0.008s

A tenfold improvement!