Avoid node.set read-only errors in Chef 11

This commit avoids the error

    Chef::Exceptions::ImmutableAttributeModification: Node attributes are read-only when you do not specify which precedence level to set.

by using the .merge() method to convert hsh from an ImmutableMash to a Mash.

Also, it's a line shorter :-)
Merge pull request #27 from satisfaction/silverware_chef_11_compatibility

Avoid (implied) node.set read-only errors in Chef 11