Fix issues with mutate gsub not working for regexp under Chef 11+

* Since Chef 11, a node object 'immutabilizes' Mash and Array attributes
  hence the value_to_str method and others in LogstashConf do not
  properly handle array items such as mutate gsub filters.  Added
  Chef::Node::ImmutableArray and Chef::Node::ImmutableMash types

* The same methods also improperly recognise Regexp objects which
  have class_to_s of 'Regexp' and not 'Regex'

* Lastly, the proper representation of a Regexp object for the
  conf file is obtained from 'source' method, not 'inspect'