🐛 Fix SequenceSet.try_convert

It's a silly mistake, but it had no test coverage.

I'm sure I did have this working at one point, but no test coverage!

lib/net/imap/sequence_set.rb

@@ -320,7 +320,7 @@ def [](first, *rest)
         # raised.
         def try_convert(obj)
           return obj if obj.is_a?(SequenceSet)
-          return nil unless respond_to?(:to_sequence_set)
+          return nil unless obj.respond_to?(:to_sequence_set)
           obj = obj.to_sequence_set
           return obj if obj.is_a?(SequenceSet)
           raise DataFormatError, "invalid object returned from to_sequence_set"

test/net/imap/test_sequence_set.rb

@@ -151,6 +151,22 @@ def compare_to_reference_set(nums, set, seqset)
     assert_raise DataFormatError do SequenceSet[Set.new] end
   end
 
+  test ".try_convert" do
+    assert_nil SequenceSet.try_convert(nil)
+    assert_nil SequenceSet.try_convert(123)
+    assert_nil SequenceSet.try_convert(12..34)
+    assert_nil SequenceSet.try_convert("12:34")
+    assert_nil SequenceSet.try_convert(Object.new)
+
+    obj = Object.new
+    def obj.to_sequence_set; SequenceSet[192, 168, 1, 255] end
+    assert_equal SequenceSet[192, 168, 1, 255], SequenceSet.try_convert(obj)
+
+    obj = Object.new
+    def obj.to_sequence_set; 192_168.001_255 end
+    assert_raise DataFormatError do SequenceSet.try_convert(obj) end
+  end
+
   test "#[non-negative index]" do
     assert_nil        SequenceSet.empty[0]
     assert_equal   1, SequenceSet[1..][0]