⚡✅ Update debug output gsub to run in O(n)

The same string transform is simple without negative lookahead by
using a block with gsub and checking `$'.empty?`.

Author: nevans

lib/net/imap.rb

@@ -2443,8 +2443,8 @@ def put_string(str)
         if @debug_output_bol
           $stderr.print("C: ")
         end
-        $stderr.print(str.gsub(/\n(?!\z)/n, "\nC: "))
-        if /\r\n\z/n.match(str)
+        $stderr.print(str.gsub(/\n/n) { $'.empty? ? $& : "\nC: " })
+        if /\n\z/n.match(str)
           @debug_output_bol = true
         else
           @debug_output_bol = false

test/net/imap/test_regexps.rb

@@ -27,9 +27,6 @@ class IMAPRegexpsTest < Test::Unit::TestCase
       exclude_map: {
         Net::IMAP => %i[BodyTypeAttachment BodyTypeExtension], # deprecated
       },
-      exclude: [
-        /\n(?!\z)/n, # TODO (in Net::IMAP#put_string)
-      ]
     ).to_h
   )