[Fixes #76] Override previous request options in concurrent issues.

[NikosRig]

[akDeveloper]

Maybe we should reset $this->options here https://github.com/akDeveloper/Aktive-Merchant/blob/master/lib/AktiveMerchant/Http/Adapter/cUrl.php#L95, after a request is completed. ex. $this->options = array().

Then, the getOptions() method should always return an empty array after a request is completed... I don't know if this is ok or breaks any backward compatibility.