Returns a function that dispatches with different strategies based on the
object in list position (last argument). If it is an array, executes [fn].
Otherwise, if it has a function with one of the given method names, it will
execute that function (functor case). Otherwise, if it is a transformer,
uses transducer [xf] to return a new transformer (transducer case).
Otherwise, it will default to executing [fn].

@private
@param {Array} methodNames properties to check for a custom implementation
@param {Function} xf transducer to initialize if object is transformer
@param {Function} fn default ramda implementation
@return {Function} A function that dispatches on object in list position
function _dispatchable(methodNames, xf, fn) {
	  return function () {
	    if (arguments.length === 0) {
	      return fn();
	    }
	    var args = Array.prototype.slice.call(arguments, 0);
	    var obj = args.pop();
	    if (!_isArray(obj)) {
	      var idx = 0;
	      while (idx < methodNames.length) {
	        if (typeof obj[methodNames[idx]] === 'function') {
	          return obj[methodNames[idx]].apply(obj, args);
	        }
	        idx += 1;
	      }
	      if (_isTransformer(obj)) {
	        var transducer = xf.apply(null, args);
	        return transducer(obj);
	      }
	    }
	    return fn.apply(this, arguments);
	  };
	}