🐛 Fix SequenceSet#merge with another SequenceSet

Most methods convert their inputs into an array of range tuples.  For
efficiency, SequenceSet inputs just use the internal `@tuples` array
directly.  Unfortunately, the internal tuples arrays were also reused,
which could cause a variety of bugs.  Fortunately, the only bug I
experienced was that adding a frozen SequenceSet would result in frozen
tuples being added to a mutable set.  But this could also result in
modifications to one SequenceSet affecting another SequenceSet!

lib/net/imap/sequence_set.rb

@@ -1342,8 +1342,8 @@ def tuple_add(tuple)
         modifying!
         min, max = tuple
         lower, lower_idx = tuple_gte_with_index(min - 1)
-        if    lower.nil?              then tuples << tuple
-        elsif (max + 1) < lower.first then tuples.insert(lower_idx, tuple)
+        if    lower.nil?              then tuples << [min, max]
+        elsif (max + 1) < lower.first then tuples.insert(lower_idx, [min, max])
         else  tuple_coalesce(lower, lower_idx, min, max)
         end
       end

test/net/imap/test_sequence_set.rb

@@ -359,6 +359,11 @@ def obj.to_sequence_set; 192_168.001_255 end
     assert_equal seqset["1:3,5,7:9"], seqset["1,3,5,7:8"].merge(seqset["2,8:9"])
     assert_equal seqset["1:*"],       seqset["5:*"].merge(1..4)
     assert_equal seqset["1:5"],       seqset["1,3,5"].merge(seqset["2,4"])
+    # when merging frozen SequenceSet
+    set = SequenceSet.new
+    set.merge SequenceSet[1, 3, 5]
+    set.merge SequenceSet[2..33]
+    assert_equal seqset[1..33], set
   end
 
   test "set - other" do