Optional SSL context.

Allows an OpenSSL context to be introduced, in case the EPP provider requires client SSL certificates. Example of how I pass the context in:

```ruby
def context
  private_key = ::OpenSSL::PKey::RSA.new File.read('private.key'), 'passphrase'
  certificate = ::OpenSSL::X509::Certificate.new File.read('public.crt')
  ssl_context = ::OpenSSL::SSL::SSLContext.new
  ssl_context.cert = certificate
  ssl_context.key  = private_key
  ssl_context
end
```