🐛 Allow SequenceSet#slice length to be negative

lib/net/imap/sequence_set.rb

@@ -2420,7 +2420,7 @@ def reverse_each_minmax_with_index(minmaxes)
       def slice_length(start, length)
         start  = Integer(start.to_int)
         length = Integer(length.to_int)
-        raise ArgumentError, "length must not be negative" if length.negative?
+        return nil if length.negative?
         stop   = start + length unless start.negative? && start.abs <= length
         slice_range(start...stop)
       end

test/net/imap/test_sequence_set.rb

@@ -487,20 +487,10 @@ def obj.to_sequence_set; 192_168.001_255 end
                  SequenceSet[1...200][100, 10000]
   end
 
-  def pend_slice_bug(what, &) = pend("#slice bug: #{what}", &)
-
-  def pend_slice_neg_len(&)
-    pend_slice_bug("allow negative length", &)
-    assert_raise_with_message(ArgumentError,
-                              "length must not be negative",
-                              &)
-  end
-
-
   test "#slice(start, length) -> nil, for negative length" do
     set = SequenceSet[1...200]
-    pend_slice_neg_len do assert_nil set[ 100, -1] end
-    pend_slice_neg_len do assert_nil set[-100, -1] end
+    assert_nil set[ 100, -1]
+    assert_nil set[-100, -1]
   end
 
   test "#slice(start, length) -> nil, for invalid start" do