allow to call count method without parameter for category type vector

[rainchen]

@zverok updated.

About the nil value issue, I'm not quite sure how to deal with. I wish to keep using the same style with count method after the vector converted to category type, e.g.:

vc.count  # => 5
vc2 = vc.to_category
vc2.count  # => 5, excepted

how about using a special :nil key for counting nil values? .e.g: vc.count(:nil)

[zverok]

About the nil value issue, I'm not quite sure how to deal with.

There is "undefined object" pattern for such cases:

UNDEFINED = Object.new.freeze # anonymous object, equal only to itself
def count category=UNDEFINED
  return .... if category == UNDEFINED

With this idiom, we know for sure that UNDEFINED would not be one of the categories in the vector, so, everything works always as expected, even for nil category.

[rainchen]

@zverok good idea

[rainchen]

@zverok All checks have passed, plz review