Don't assume exception has a response

The wrapped OpenSSL exceptions don't have a response:

    #<Faraday::SSLError wrapped=#<OpenSSL::SSL::SSLError: SSL_connect returned=1 errno=0 state=error: certificate verify failed>>

Follow up to #585.

Author: drewish

lib/raven/transports/http.rb

@@ -29,7 +29,9 @@ def send_event(auth_header, data, options = {})
         end
       rescue Faraday::ClientError => ex
         error_info = ex.message
-        error_info += " Error in headers is: #{ex.response[:headers]['x-sentry-error']}" if ex.response[:headers]['x-sentry-error']
+        if ex.response && ex.response[:headers]['x-sentry-error']
+          error_info += " Error in headers is: #{ex.response[:headers]['x-sentry-error']}"
+        end
         raise Raven::Error, error_info
       end