model: Avoid frozen literal warning

[jlduran]

Avoid warnings about mutating a literal, as they will be frozen in the future.

[karlnaden]

github is flaking on me and not allowing me to add additional comments. There are two others beyond the suggestion I made both in the vein of make the code a little cleaner with clearer variable names:

    def check_binding_uri(uri_with_version, value)
      valid = false
      # Strip off the |4.0.0 or |4.0.1 or |2014-03-26 or similar from the ends of URLs
      uri = uri_with_version&.gsub(/\|[A-Za-z0-9.-]*/, '')

  def self.get(raw_key, hash)
    return @@context if ['$context', '$resource'].include?(raw_key)
    return @@parent if raw_key == '$parent'
    return 'http://unitsofmeasure.org' if raw_key == '%ucum'
    return 'http://snomed.info/sct' if raw_key == '%sct'
    return 'http://loinc.org' if raw_key == '%loinc'
    return raw_key.gsub(/\A\'|\'\Z/, '') if orig_key.start_with?("'") && orig_key.end_with?("'")
    key = raw_key.gsub(/\A"|"\Z/, '') # remove quotes around path if they exist