Use prepend when defined instead of alias_method_chain

This is to get the ball rolling on upgrading to Module#prepend,
since `alias_method_chain` is deprecated in Rails 5.0.

This PR is probably not an ideal implementation.

A simpler migration would be to replace alias_method_chain with two
alias_methods.

Opening the discussion…

lib/web_console/extensions.rb

@@ -1,19 +1,40 @@
-ActionDispatch::DebugExceptions.class_eval do
-  def render_exception_with_web_console(env, exception)
-    render_exception_without_web_console(env, exception).tap do
-      error = ActionDispatch::ExceptionWrapper.new(env, exception).exception
+if defined?(Module.prepend) && Rails.version >= '4'
+  module RenderExceptionsWithWebConsole
+    def render_exception(env, exception)
+      super.tap do
+        error = ActionDispatch::ExceptionWrapper.new(env, exception).exception
 
-      # Get the original exception if ExceptionWrapper decides to follow it.
-      env['web_console.exception'] = error
+        # Get the original exception if ExceptionWrapper decides to follow it.
+        env['web_console.exception'] = error
 
-      # ActionView::Template::Error bypass ExceptionWrapper original
-      # exception following. The backtrace in the view is generated from
-      # reaching out to original_exception in the view.
-      if error.is_a?(ActionView::Template::Error)
-        env['web_console.exception'] = error.original_exception
+        # ActionView::Template::Error bypass ExceptionWrapper original
+        # exception following. The backtrace in the view is generated from
+        # reaching out to original_exception in the view.
+        if error.is_a?(ActionView::Template::Error)
+          env['web_console.exception'] = error.original_exception
+        end
       end
     end
   end
 
-  alias_method_chain :render_exception, :web_console
+  module ActionDispatch
+    class DebugExceptions
+      prepend RenderExceptionsWithWebConsole
+    end
+  end
+else
+  # Legacy code that duplicates the module above, using `alias_method_chain`.
+  # TODO: Remove me when support for Ruby < 2 && Rails < 4 is dropped.
+  ActionDispatch::DebugExceptions.class_eval do
+    def render_exception_with_web_console(env, exception)
+      render_exception_without_web_console(env, exception).tap do
+        error = ActionDispatch::ExceptionWrapper.new(env, exception).exception
+        env['web_console.exception'] = error
+        if error.is_a?(ActionView::Template::Error)
+          env['web_console.exception'] = error.original_exception
+        end
+      end
+    end
+    alias_method_chain :render_exception, :web_console
+  end
 end