[rackspace|compute] Handle malformed API responses.

When deleting a Cloud Network, the API returns a plain text
response even when the Content-Type is set to
application/json. In the event of a JSON parsing error, this
change sets the request body to an empty hash.

lib/fog/rackspace/compute_v2.rb

@@ -127,8 +127,13 @@ module Fog
           rescue Excon::Errors::HTTPStatusError => error
             raise ServiceError.slurp error
           end
+
           unless response.body.empty?
-            response.body = Fog::JSON.decode(response.body)
+            begin
+              response.body = Fog::JSON.decode(response.body)
+            rescue MultiJson::DecodeError => e
+              response.body = {}
+            end
           end
           response
         end