code: public Jar setMapAttribute(String name, Map<String, ?> values) {
          return setAttribute(name, join(values));
      }
docs: Sets an attribute in the main section of the manifest to a map.
  The map entries will be joined with a single whitespace character, and each key-value pair will be joined with a '='.
  
  @param name   the attribute's name
  @param values the attribute's value
  @return {@code this}
  @throws IllegalStateException if entries have been added or the JAR has been written prior to calling this methods.