Remove array key assignment

According to
http://docs.puppetlabs.com/puppet/2.7/reference/lang_datatypes.html#significant-bugs-mutability-1
it is incorrect to mutate hashes by assigning directly to keys. This is
enforced in the future parser introduced in Puppet 3.2.0 and is reported
in #213
Merge pull request #215 from hunner/hash_mute

Remove array key assignment