🔧 Add Config#load_defaults

[nevans]

• Depends on
  • ✨ Add Config methods: #to_h, #update, and #with #300
  • 🔧 Add versioned defaults #302
• Completes Add Net::IMAP::Config class for global and client-scoped config #288.

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

Config#load_defaults resets the current config to behave like the versioned default configuration for the requested version. (See #302.)

For example, #load_defaults can be used to globally behave more like a specific version of net-imap:

client = Net::IMAP.new(hostname)
client.config.sasl_ir              # => true
Net::IMAP.config.load_defaults 0.3
client.config.sasl_ir              # => false