Allow get_cookies to find comma separated cookies

While reviewing #3463 I've found the actual ```Rex::Proto::Http::Response::get_cookies``` is failing to find comma separated cookies, as set by applications like Splunk 5.0.1 on Windows 7 (32 bits). Example:

```
Set-Cookie: cval=880350187, session_id_8000=83466b1a1a7a27ce13d35f78155d40ca3a1e7a28; expires=Mon, 07 Jul 2014 20:09:28 GMT; httponly; Path=/, uid=348637C4-9B10-485A-BFA9-5E892432FCFD; expires=Fri, 05-Jul-2019 20:09:28 GMT
```

In the case above, ```get_cookies``` fails to find the ```cval``` cookie, because it's separated with a comma from ```session_id_8000```:

```
msf > irb
[*] Starting IRB shell...

>> res = Rex::Proto::Http::Response.new
=> HTTP/1.1 200 OK
Content-Length: 0
>>
?> response = <<-HEREDOC
HTTP/1.1 200 OK
Expires: Thu, 26 Oct 1978 00:00:00 GMT
Content-Length: 8556
Server: CherryPy/3.1.2
Date: Sun, 06 Jul 2014 20:09:28 GMT
Cache-Control: no-store, max-age=0, no-cache, must-revalidate
Content-Type: text/html;charset=utf-8
Set-Cookie: cval=880350187, session_id_8000=83466b1a1a7a27ce13d35f78155d40ca3a1e7a28; expires=Mon, 07 Jul 2014 20:09:28 GMT; httponly; Path=/, uid=348637C4-9B10-485A-BFA9-5E892432FCFD; expires=Fri, 05-Jul-2019 20:09:28 GMT
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<!--[if lt IE 7]> <html xmlns="http://www.w3.org/1999/xhtml" xmlns:s="http://www.splunk.com/xhtml-extensions/1.0" xml:lang="en" lang="en" class="no-js lt-ie9 lt-ie8 lt-
HEREDOC
=> "HTTP/1.1 200 OK\nExpires: Thu, 26 Oct 1978 00:00:00 GMT\nContent-Length: 8556\nServer: CherryPy/3.1.2\nDate: Sun, 06 Jul 2014 20:09:28 GMT\nCache-Control: no-store, max-age=0, no-cache, must-revalidate\nContent-Type: text/html;charset=utf-8\nSet-Cookie: cval=880350187, session_id_8000=83466b1a1a7a27ce13d35f78155d40ca3a1e7a28; expires=Mon, 07 Jul 2014 20:09:28 GMT; httponly; Path=/, uid=348637C4-9B10-485A-BFA9-5E892432FCFD; expires=Fri, 05-Jul-2019 20:09:28 GMT\n\n<!DOCTYPE html PUBLIC \"-//W3C//DTD XHTML 1.0 Strict//EN\" \"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd\">\n<!--[if lt IE 7]> <html xmlns=\"http://www.w3.org/1999/xhtml\" xmlns:s=\"http://www.splunk.com/xhtml-extensions/1.0\" xml:lang=\"en\" lang=\"en\" class=\"no-js lt-ie9 lt-ie8 lt-\n"
>>
?> res.parse(response)
=> 2
>>
?> res.get_cookies
=> "session_id_8000=83466b1a1a7a27ce13d35f78155d40ca3a1e7a28; uid=348637C4-9B10-485A-BFA9-5E892432FCFD;"
```

This patch tries to find the comma separated cookies:

```
msf > irb
[*] Starting IRB shell...

>> res = Rex::Proto::Http::Response.new
=> HTTP/1.1 200 OK
Content-Length: 0


>>
?> response = <<-HEREDOC
HTTP/1.1 200 OK
Expires: Thu, 26 Oct 1978 00:00:00 GMT
Content-Length: 8556
Server: CherryPy/3.1.2
Date: Sun, 06 Jul 2014 20:09:28 GMT
Cache-Control: no-store, max-age=0, no-cache, must-revalidate
Content-Type: text/html;charset=utf-8
Set-Cookie: cval=880350187, session_id_8000=83466b1a1a7a27ce13d35f78155d40ca3a1e7a28; expires=Mon, 07 Jul 2014 20:09:28 GMT; httponly; Path=/, uid=348637C4-9B10-485A-BFA9-5E892432FCFD; expires=Fri, 05-Jul-2019 20:09:28 GMT
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<!--[if lt IE 7]> <html xmlns="http://www.w3.org/1999/xhtml" xmlns:s="http://www.splunk.com/xhtml-extensions/1.0" xml:lang="en" lang="en" class="no-js lt-ie9 lt-ie8 lt-
HEREDOC
=> "HTTP/1.1 200 OK\nExpires: Thu, 26 Oct 1978 00:00:00 GMT\nContent-Length: 8556\nServer: CherryPy/3.1.2\nDate: Sun, 06 Jul 2014 20:09:28 GMT\nCache-Control: no-store, max-age=0, no-cache, must-revalidate\nContent-Type: text/html;charset=utf-8\nSet-Cookie: cval=880350187, session_id_8000=83466b1a1a7a27ce13d35f78155d40ca3a1e7a28; expires=Mon, 07 Jul 2014 20:09:28 GMT; httponly; Path=/, uid=348637C4-9B10-485A-BFA9-5E892432FCFD; expires=Fri, 05-Jul-2019 20:09:28 GMT\n\n<!DOCTYPE html PUBLIC \"-//W3C//DTD XHTML 1.0 Strict//EN\" \"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd\">\n<!--[if lt IE 7]> <html xmlns=\"http://www.w3.org/1999/xhtml\" xmlns:s=\"http://www.splunk.com/xhtml-extensions/1.0\" xml:lang=\"en\" lang=\"en\" class=\"no-js lt-ie9 lt-ie8 lt-\n"
>>
?> res.parse(response)
=> 2
>>
?> res.get_cookies
=> "cval=880350187; session_id_8000=83466b1a1a7a27ce13d35f78155d40ca3a1e7a28; uid=348637C4-9B10-485A-BFA9-5E892432FCFD;"
```

This regular expression isn't perfect to find cookies still, specially because is not able to find correctly values including commas. But hopefully it's better than before. Feedback is welcome.