actioncontroller: fix render_to_string method signature

The block is optional.

Author: rhiroe

gems/actionpack/6.0/_test/test.rb

@@ -48,3 +48,5 @@ def fetch
     post :quux
   end
 end
+
+ActionController::Base.new.render_to_string(template: 'foo', locals: { bar: 'baz' })

gems/actionpack/6.0/actioncontroller.rbs

@@ -276,3 +276,9 @@ module ActionController
              | (untyped key, untyped args) -> untyped
   end
 end
+
+module ActionController
+  module Rendering
+    def render_to_string: (*untyped args) ?{ () -> untyped } -> ::String
+  end
+end

gems/actionpack/6.0/actionpack-generated.rbs

@@ -511,18 +511,6 @@ module AbstractController
     # sticks the result in <tt>self.response_body</tt>.
     def render: (*untyped args) { () -> untyped } -> untyped
 
-    # Raw rendering of a template to a string.
-    #
-    # It is similar to render, except that it does not
-    # set the +response_body+ and it should be guaranteed
-    # to always return a string.
-    #
-    # If a component extends the semantics of +response_body+
-    # (as ActionController extends it to be anything that
-    # responds to the method each), this method needs to be
-    # overridden in order to still return a string.
-    def render_to_string: (*untyped args) { () -> untyped } -> untyped
-
     # Performs the actual template rendering.
     def render_to_body: (?::Hash[untyped, untyped] options) -> nil
 
@@ -2748,9 +2736,6 @@ module ActionController
 
     def render: (*untyped args) -> untyped
 
-    # Overwrite render_to_string because body can now be set to a Rack body.
-    def render_to_string: () -> untyped
-
     def render_to_body: (?::Hash[untyped, untyped] options) -> untyped
 
     private