🔧 Set responses_without_block to frozen_dup in 0.6

This seems like a better API than _always_ crashing.  Now that we have a
config option for it, it should become the default in v0.6.

lib/net/imap.rb

@@ -2534,11 +2534,11 @@ def idle_done
     #       Prints a warning and returns the mutable responses hash.
     #       <em>This is not thread-safe.</em>
     #
-    #     [+:frozen_dup+</em>]
+    #     [+:frozen_dup+ <em>(planned default for +v0.6+)</em>]
     #       Returns a frozen copy of the unhandled responses hash, with frozen
     #       array values.
     #
-    #     [+:raise+ <em>(planned future default)</em>]
+    #     [+:raise+]
     #       Raise an +ArgumentError+ with the deprecation warning.
     #
     # For example:

lib/net/imap/config.rb

@@ -237,7 +237,7 @@ def self.[](config)
       #   Prints a warning and returns the mutable responses hash.
       #   <em>This is not thread-safe.</em>
       #
-      # [+:frozen_dup+</em>]
+      # [+:frozen_dup+ <em>(planned default for +v0.6+)</em>]
       #   Returns a frozen copy of the unhandled responses hash, with frozen
       #   array values.
       #
@@ -246,7 +246,7 @@ def self.[](config)
       #
       #   <em>(+:frozen_dup+ config option was added in +v0.4.17+)</em>
       #
-      # [+:raise+ <em>(planned future default)</em>]
+      # [+:raise+]
       #   Raise an ArgumentError with the deprecation warning.
       #
       # Note: #responses_without_args is an alias for #responses_without_block.
@@ -363,9 +363,10 @@ def defaults_hash
       version_defaults[:current] = Config[0.4]
       version_defaults[:next]    = Config[0.5]
 
-      version_defaults[:future]  = Config[0.5].dup.update(
-        responses_without_block: :raise,
+      version_defaults[0.6]      = Config[0.5].dup.update(
+        responses_without_block: :frozen_dup,
       ).freeze
+      version_defaults[:future]  = Config[0.6]
 
       version_defaults.freeze
     end

test/net/imap/test_config.rb

@@ -191,6 +191,7 @@ class ConfigTest < Test::Unit::TestCase
     assert_same Config.global,  Config.new(Config.global).parent
     assert_same Config[0.4],    Config.new(0.4).parent
     assert_same Config[0.5],    Config.new(:next).parent
+    assert_same Config[0.6],    Config.new(:future).parent
     assert_equal true, Config.new({debug: true}, debug: false).parent.debug?
     assert_equal true, Config.new({debug: true}, debug: false).parent.frozen?
   end