Avoid "method redefined" warnings in Delegator

When extensions (like sinatra-contrib) are registered, they often
re-delegate methods that are already defined on the Delegator
(e.g., `get`, `post`, `config_file`). This causes noisy "method redefined"
warnings at runtime.

This change adds a check to `Delegator.delegate` to skip defining the
method if it already exists. Since the delegation logic is identical
(forwarding to `Delegator.target`), skipping the redefinition is safe
and eliminates the warnings.

Author: dentarg

lib/sinatra/base.rb

@@ -2103,6 +2103,8 @@ def self.register(*extensions, &block) # :nodoc:
   module Delegator # :nodoc:
     def self.delegate(*methods)
       methods.each do |method_name|
+        next if method_defined?(method_name) || private_method_defined?(method_name)
+
         define_method(method_name) do |*args, &block|
           return super(*args, &block) if respond_to? method_name