Adding config to openssl_pkey_export()

Closed pull request by mistake (https://github.com/zendframework/zf2/pull/3927)

openssl_pkey_export throws strange errors - Adding the optional config parameter (with config path) fixes the problem. 

![Screen Shot 2013-03-01 at 14 51 24](https://f.cloud.github.com/assets/1678209/216190/9457cb8c-84b7-11e2-89c7-d346a1730432.png)