Fixed ClientCertificate NotImplementedException on mono runtime.

In mono there is no setter for the ClientCertificates property on the WebRequest
class, and therefore it throws a NotSupportedException. However, if the collection
is empty upon access a new instance is added and returned, thus, one can simply
add the current certificates to the request's certificate collection.
Merge pull request #396 from nover/master

Fixed ClientCertificate NotImplementedException on mono runtime.