Support exceptions without a #response method

Hi! 

While trying to use `webmock` with this gem, I ran into this issue today:

```
  1) PaymentGateway PaymentGateway::MercadoPagoGateway#state asynchronous credential check which actually runs should mark the state as confirmed
     Failure/Error: CheckMercadoPagoCredentialsJob.new.perform(@mercado_pago.id)
     NoMethodError:
       undefined method `response' for #<WebMock::NetConnectNotAllowedError:0x0000010cb73b28>
     # ./app/models/payment_gateway/mercado_pago_gateway.rb:130:in `check_credentials'
     # ./app/jobs/check_mercado_pago_credentials_job.rb:21:in `perform'
     # ./spec/models/payment_gateway/payment_gateway_spec.rb:32:in `block (4 levels) in <top (required)>'
```

I expected to see something like this:

```
  1) PaymentGateway PaymentGateway::MercadoPagoGateway#state asynchronous credential check which actually runs should mark the state as confirmed
     Failure/Error: CheckMercadoPagoCredentialsJob.new.perform(@mercado_pago.id)
     WebMock::NetConnectNotAllowedError:
       Real HTTP connections are disabled. Unregistered request: POST https://api.mercadolibre.com/oauth/token with body 'client_id=1234&client_secret=SECRET&grant_type=client_credentials' with headers {'Accept'=>'application/json', 'Accept-Encoding'=>'gzip;q=1.0,deflate;q=0.6,identity;q=0.3', 'Content-Type'=>'application/x-www-form-urlencoded', 'User-Agent'=>'Ruby'}
       
       You can stub this request with the following snippet:
       
       stub_request(:post, "https://api.mercadolibre.com/oauth/token").
         with(:body => {"client_id"=>"1234", "client_secret"=>"SECRET", "grant_type"=>"client_credentials"},
              :headers => {'Accept'=>'application/json', 'Accept-Encoding'=>'gzip;q=1.0,deflate;q=0.6,identity;q=0.3', 'Content-Type'=>'application/x-www-form-urlencoded', 'User-Agent'=>'Ruby'}).
         to_return(:status => 200, :body => "", :headers => {})
       
       ============================================================
```

This pull request fixes this issue. If an Exception doesn't have the `#response` method,
it will work.

Check it out.

Thanks!