maintain blocks passed to 'perform' in redirects

Blocks passed to requests only get executed on the first HTTP request and don't get executed on future requests caused by a redirect.

For instance:
```ruby
HTTParty.get("http://google.com") { |chunk| puts "Got a chunk: #{chunk}\n\n" }
```
will only print out the lines for the 301 response, but will not be run for the following request.