✨ Add support for UNSELECT extension (RFC3691)

This is needed for IMAP4rev2 #12.

Fixes #40.

lib/net/imap.rb

@@ -796,6 +796,20 @@ def close
       send_command("CLOSE")
     end
 
+    # Sends an {UNSELECT command [IMAP4rev2
+    # §6.4.2]}[https://www.rfc-editor.org/rfc/rfc9051#section-6.4.2] to free the
+    # session resources for a mailbox and return to the "_authenticated_" state.
+    # This is the same as #close, except that <tt>\\Deleted</tt> messages are
+    # not removed from the mailbox.
+    #
+    # ===== Capabilities
+    #
+    # The server's capabilities must include +UNSELECT+
+    # [RFC3691[https://tools.ietf.org/html/rfc3691]].
+    def unselect
+      send_command("UNSELECT")
+    end
+
     # Sends a EXPUNGE command to permanently remove from the currently
     # selected mailbox all messages that have the \Deleted flag set.
     def expunge

test/net/imap/test_imap.rb

@@ -915,6 +915,27 @@ def test_close
     end
   end
 
+  def test_unselect
+    requests = Queue.new
+    port = yields_in_test_server_thread do |sock, gets|
+      requests.push(gets[])
+      sock.print("RUBY0001 OK UNSELECT completed\r\n")
+      requests.push(gets[])
+      "RUBY0002"
+    end
+    begin
+      imap = Net::IMAP.new(server_addr, :port => port)
+      resp = imap.unselect
+      assert_equal(["RUBY0001", "UNSELECT", ""], requests.pop)
+      assert_equal([Net::IMAP::TaggedResponse, "RUBY0001", "OK"],
+                   [resp.class, resp.tag, resp.name])
+      imap.logout
+      assert_equal(["RUBY0002", "LOGOUT", ""], requests.pop)
+    ensure
+      imap.disconnect if imap
+    end
+  end
+
   private
 
   def imaps_test