Fix for Redis EVAL / EVALSHA

phpredis casts nil responses to bool(false), whereas HHVM returns NULL for both
false and nil return values. For example:

    <?php
    $redis = new Redis();
    $redis->connect( '127.0.0.1' );
    var_dump( $redis->evaluate( 'return' ) );
    var_dump( $redis->evaluate( 'return false' ) );

With Zend / phpredis, the output is:

    bool(false)
    bool(false)

With HHVM, it's:

    NULL
    NULL

This patch adds a fix and a test.