Add test and note for ULID#<=> signature

lib/ulid.rb

@@ -392,7 +392,7 @@ def hash
     [ULID, @integer].hash
   end
 
-  # @return [Integer, nil]
+  # @return [-1, 0, 1, nil]
   def <=>(other)
     (ULID === other) ? (@integer <=> other.to_i) : nil
   end

sig/ulid.rbs

@@ -411,7 +411,12 @@ class ULID < Object
   # #=> true
   # ```
   #
-  # To be precise, this sorting unaffected with `case sensitive or not` and might handle [ulid/spec#57](https://github.com/ulid/spec/pull/57) in future. So preferable than `lexicographically sortable` in actual case.
+  # To be precise, this sorting unaffected with `case sensitive or not` and might handle [ulid/spec#57](https://github.com/ulid/spec/pull/57) in future.
+  # So preferable than `lexicographically sortable` in actual case.
+  #
+  # This returns -1 | 0 | 1 for ULIDs. However defined as returning Integer. It is caused on ruby/rbs current definition.
+  #   https://github.com/ruby/ruby/blob/cd34f56d450f2310cceaf4c5f34d23eddfda58e8/numeric.c#L4646-L4660
+  #   https://github.com/ruby/rbs/blob/14abbbae8885a09a2ed82de2ef31d67a9c0a108d/core/integer.rbs#L461-L462
   #
   def <=>: (ULID other) -> Integer
          | (untyped other) -> nil

test/core/test_ulid_instance.rb

@@ -123,6 +123,11 @@ def test_eqq
 
   def test_cmp
     ulid = ULID.sample
+    assert_equal(1, ulid <=> ulid.pred)
+    assert_equal(1, ulid <=> ulid.pred.pred.pred)
+    assert_equal(0, ulid <=> ULID.parse(ulid.to_s))
+    assert_equal(-1, ulid <=> ulid.next)
+    assert_equal(-1, ulid <=> ulid.next.next.next)
     [nil, BasicObject.new, '01ARZ3NDEKTSV4RRFFQ69G5FAV', 42, Time.now].each do |not_comparable|
       assert_nil(ulid <=> not_comparable)
     end