🔧 Add Config#load_defaults

This is inspired by Rails::Application::Configuration#load_defaults in
rails.

`#load_defaults` resets the current config to behave like the versioned
default configuration for that version.  `#parent` will not be changed.

Some config attributes default to inheriting from their `#parent` (which
is usually Config.global) and are left unchanged, for example: `#debug`.

Author: nevans

lib/net/imap/config.rb

@@ -87,6 +87,12 @@ class IMAP
     #   Net::IMAP.debug = true
     #   client.config.debug?  # => true
     #
+    # Use #load_defaults to globally behave like a specific version:
+    #   client = Net::IMAP.new(hostname)
+    #   client.config.sasl_ir              # => true
+    #   Net::IMAP.config.load_defaults 0.3
+    #   client.config.sasl_ir              # => false
+    #
     # === Named defaults
     # In addition to +x.y+ version numbers, the following aliases are supported:
     #
@@ -270,6 +276,23 @@ def with(**attrs)
         block_given? ? yield(copy) : copy
       end
 
+      # :call-seq: load_defaults(version) -> self
+      #
+      # Resets the current config to behave like the versioned default
+      # configuration for +version+.  #parent will not be changed.
+      #
+      # Some config attributes default to inheriting from their #parent (which
+      # is usually Config.global) and are left unchanged, for example: #debug.
+      #
+      # See Config@Versioned+defaults and Config@Named+defaults.
+      def load_defaults(version)
+        [Numeric, Symbol, String].any? { _1 === version } or
+          raise ArgumentError, "expected number or symbol, got %p" % [version]
+        config = Config[version]
+        defaults = config.to_h.reject {|k,v| DEFAULT_TO_INHERIT.include?(k) }
+        update(**defaults)
+      end
+
       # :call-seq: to_h -> hash
       #
       # Returns all config attributes in a hash.

test/net/imap/test_config.rb

@@ -332,4 +332,27 @@ class ConfigTest < Test::Unit::TestCase
     assert_equal [11, 5, true], vals
   end
 
+  test "#load_defaults" do
+    config = Config.global.load_defaults 0.3
+    assert_same Config.global, config
+    assert_same true,  config.inherited?(:debug)
+    assert_same false, config.inherited?(:sasl_ir)
+    assert_same false, config.sasl_ir
+    # does not _reset_ default
+    config.debug = true
+    Config.global.load_defaults 0.3
+    assert_same false, config.inherited?(:debug)
+    assert_same true,  config.debug?
+    # does not change parent
+    child           = Config.global.new
+    grandchild      = child.new
+    greatgrandchild = grandchild.new
+    child.load_defaults :current
+    grandchild.load_defaults :next
+    greatgrandchild.load_defaults :future
+    assert_same Config.global, child.parent
+    assert_same child, grandchild.parent
+    assert_same grandchild, greatgrandchild.parent
+  end
+
 end