Don't assume ActionMailer is available (#608)

If the Rails app that uses this gem doesn't include the
ActionMailer railtie, the gem raises this error:

`uninitialized constant ReactOnRailsHelper::ActionMailer`

This change introduces a check that the ActionMailer
constant is defined before setting the 'inMailer' option.

I'm not quite sure how to test this, so if anyone has
ideas, that would be helpful.

app/helpers/react_on_rails_helper.rb

@@ -345,7 +345,7 @@ def initialize_redux_stores
   def rails_context(server_side:)
     @rails_context ||= begin
       result = {
-        inMailer: controller.present? && controller.is_a?(ActionMailer::Base),
+        inMailer: in_mailer?,
         # Locale settings
         i18nLocale: I18n.locale,
         i18nDefaultLocale: I18n.default_locale
@@ -400,4 +400,11 @@ def send_tag_method(tag_method_name, args)
     options = args.delete_if { |key, _value| %i(hot static).include?(key) }
     send(tag_method_name, *assets, options) unless assets.empty?
   end
+
+  def in_mailer?
+    return false unless controller.present?
+    return false unless defined?(ActionMailer::Base)
+
+    controller.is_a?(ActionMailer::Base)
+  end
 end