Small tweak to how the rs_params hash is updated with cookies

RightApi returned nil for (all?) get requests after login.

Tested with

    ruby 1.8.7 (2012-02-08 patchlevel 358) [universal-darwin12.0]
    rest-client (1.6.7)

Test case:

```ruby
#!/usr/bin/ruby
require 'RightAPI'
api = RightAPI.new
api.login(:username => 'myusername', :password => 'mypassword', :account => '1234')
# login success
puts api.send("servers")
```

Output before fix:

```
    nil
```

Output after fix:

```xml
<?xml version="1.0" encoding="UTF-8"?>
<servers type="array">
  <server type="Server">
...
```