♻️ Anchor literal regexp to the end of the buffer

This also allows us to check against the concatenated buffer, rather
than the smaller line buffer.  That distinction doesn't really matter
now, since we always read an entire line at once.  But it will matter if
we read partial lines.

Author: nevans

lib/net/imap.rb

@@ -3437,7 +3437,7 @@ def get_response
         s = @sock.gets(CRLF)
         break unless s
         buff.concat(s)
-        if /\{(\d+)\}\r\n/n =~ s
+        if /\{(\d+)\}\r\n\z/n =~ buff
           s = @sock.read($1.to_i)
           buff.concat(s)
         else