Ensure that UTF8 is used by rack

Sadly Ruby 1.9 does not come with UTF-8 encoding as default but uses US-ASCII instead. This produces errors when trying to compile the catalog
```
Error: Could not retrieve catalog from remote server: Error 400 on SERVER: invalid byte sequence in US-ASCII at /etc/puppet/environments/migration/modules/demo/manifests/init.pp:1 on node alex-dev-vm-demo.ch
Warning: Not using cache on failed catalog
```
The workaround to limit contain this for puppet only seems to do in the Rack configuration.  You can find the discussion [here] (https://projects.puppetlabs.com/issues/20897). It is not parametrized while it is the default encoding for all other Ruby versions and having this change standarizes the setup.

Debian wheezy ships with Ruby 1.9.3 so I am affected by this problem.