Fix IMAP search issues

The code had several issues with existing IMAP UID search:
* net-imap <= v0.3 returns `nil` when the server returns nothing.
  (>= v0.4 returns an empty array when the server returns nothing.)
* net-imap v0.4.8 and v0.4.9 return a _frozen_ `SearchResult`.
  `SearchResult` inherits from `Array`, but because it's frozen,
  mutating it with `reverse!` results in an exception.
  See ruby/net-imap#262.
  (v0.4.10 and above doesn't freeze SearchResult.)
* newer net-imap (>= 0.5) will support servers that send `ESEARCH`
  results, but those will be returned as an `ESearchResult` struct
  (probably frozen).  `IMAP4rev1` servers shouldn't return `ESEARCH`
  unless the client has activated that extension, but `IMAP4rev2`
  servers will _always_ return `ESEARCH`.
* RFC3501 says nothing about sorting the UIDs that come back from
  `UID SEARCH`.  Almost all servers do return sorted UIDs, most of the
  time.  But it isn't 100% reliable.

The fix is simple:
* use `#to_a` to convert both `nil` and `ESearchResult` into an array.
* use `#sort` to ensure the UIDs are sorted.  This also returns a new
  array, without mutating the original (which may be frozen).

lib/mail/network/retriever_methods/imap.rb

@@ -75,7 +75,10 @@ def find(options=nil, &block)
 
       start do |imap|
         options[:read_only] ? imap.examine(options[:mailbox]) : imap.select(options[:mailbox])
+
         uids = imap.uid_search(options[:keys], options[:search_charset])
+          .to_a # older net-imap may return nil, newer may return ESearchResult struct
+          .sort # RFC3501 does _not_ require UIDs to be returned in order
         uids.reverse! if options[:what].to_sym == :last
         uids = uids.first(options[:count]) if options[:count].is_a?(Integer)
         uids.reverse! if (options[:what].to_sym == :last && options[:order].to_sym == :asc) ||