Fix IMAP search issues

[nevans]

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 https://github.com/ruby/net-imap/issues/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 of UIDs.
• use #sort to ensure the UIDs are sorted. This also returns a new array, without mutating the original (which may be frozen).

[nevans]

Any thoughts? Would you like me to add some tests to simulate the potential return values?

• unsorted (depends on the server, any version of net-imap)
• nil (returned by net-imap <= 0.3)
• frozen (net-imap, >= 0.4.8, <= 0.4.9, and maybe >= 0.6 or >= 0.7)
• an extended search result object which implements to_a for backward-compatibility (IMAP4rev2 servers, net-imap >= 0.5)

[eval]

Looks mergeable to me 👍🏻

[eval]

Thanks for the extensive writeup!