🐛 Fix SequenceSet[input] for a SequenceSet input

I'm surprised there was no test for this!

Author: nevans

lib/net/imap/sequence_set.rb

@@ -297,7 +297,7 @@ class << self
         # Use ::new to create a mutable or empty SequenceSet.
         def [](first, *rest)
           if rest.empty?
-            if first.is_a?(SequenceSet) && set.frozen? && set.valid?
+            if first.is_a?(SequenceSet) && first.frozen? && first.valid?
               first
             else
               new(first).validate.freeze

test/net/imap/test_sequence_set.rb

@@ -103,6 +103,11 @@ def compare_to_reference_set(nums, set, seqset)
     assert_raise DataFormatError do SequenceSet.new Set[1..20]   end
   end
 
+  test ".[frozen SequenceSet] returns that SequenceSet" do
+    frozen_seqset = SequenceSet[123..456]
+    assert_same frozen_seqset, SequenceSet[frozen_seqset]
+  end
+
   test ".new, input may be empty" do
     assert_empty SequenceSet.new
     assert_empty SequenceSet.new []