🐛 Fix SequenceSet#append when its @string is nil

[nevans]

When a SequenceSet is not normalized, it stores its string in @string. When it is normalized, @string is still used as a cache for @string.

When @string isn't set, but the sequence set is not empty, #append would generate the normalized string after adding the new numbers, and then append the new numbers to the end of the string. For example:

SequenceSet.new.append(5).append(1).string
# in net-imap v0.5.5 => "1,5,1"
# with this bugfix   => "1,5"

This PR forces the string to be generated prior to adding the new numbers, so they can be appended to the end.