nested_value_set -- parameter list change

* 'keys_are_syms' becomes 'transform_keys'
* if neither is used, behavior is unchanged
  (incoming keys are forced to symbols)
* old and new parameters are both optional
    * nested_value_set(hash, keys, value, keys_are_syms=true)
    * nested_value_set(hash, keys, value, transform_keys=:symbol)
* old keys_are_syms  semantics:
    * absent -- incoming keys are forced to symbols
    * true   -- incoming keys are forced to symbols
    * false  -- incoming keys are forced to strings
* new transform_keys semantics:
    * absent  -- incoming keys are forced to symbols
    * :symbol -- incoming keys are forced to symbols
    * :string -- incoming keys are forced to strings
    * :false or false --
                 incoming keys are not changed
* change motivated by Gyoku XML serializer, which treats
  the types of keys in a ruby hash as meaningful, and also
  requires certain keys to be symbols (such as :'order!')