Make sure that we don't perform in-place mutation on SafeBuffer string

This will make sure `render :inline` is working.

Closes #1633

actionpack/lib/action_view/template.rb

@@ -218,6 +218,9 @@ module ActionView
         method_name = self.method_name
 
         if source.encoding_aware?
+          # Avoid performing in-place mutation for SafeBuffer
+          @source = source.to_str if source.html_safe?
+
           # Look for # encoding: *. If we find one, we'll encode the
           # String in that encoding, otherwise, we'll use the
           # default external encoding.

actionpack/test/template/template_test.rb

@@ -172,5 +172,10 @@ class TestERBTemplate < ActiveSupport::TestCase
     ensure
       silence_warnings { Encoding.default_external = old }
     end
+
+    def test_render_inline_safebuffer_should_not_raise_error
+      @template = new_template("Hello".html_safe)
+      render
+    end
   end
 end