backport #8662, charset should not be appended for `head` responses

      1) Failure:
    test_head_created_with_image_png_content_type(RenderTest) [test/controller/render_test.rb:1238]:
    Expected: "image/png"
      Actual: "image/png; charset=utf-8"

2 files changed, 8 insertions, 3 deletions

diff --git a/actionpack/lib/action_controller/metal/head.rb b/actionpack/lib/action_controller/metal/head.rb
index 671053566d..4c61bbd6dc 100644
--- a/actionpack/lib/action_controller/metal/head.rb
+++ b/actionpack/lib/action_controller/metal/head.rb
@@ -29,6 +29,7 @@ module ActionController
       self.status = status
       self.location = url_for(location) if location
       self.content_type = content_type || (Mime[formats.first] if formats)
+      self.response.charset = false if self.response
       self.response_body = " "
     end
   end
diff --git a/actionpack/lib/action_dispatch/http/response.rb b/actionpack/lib/action_dispatch/http/response.rb
index 5797c63924..60d97c5e77 100644
--- a/actionpack/lib/action_dispatch/http/response.rb
+++ b/actionpack/lib/action_dispatch/http/response.rb
@@ -56,7 +56,7 @@ module ActionDispatch # :nodoc:
     CONTENT_TYPE = "Content-Type".freeze
     SET_COOKIE   = "Set-Cookie".freeze
     LOCATION     = "Location".freeze
- 
+
     cattr_accessor(:default_charset) { "utf-8" }
 
     include Rack::Response::Helpers
@@ -195,12 +195,16 @@ module ActionDispatch # :nodoc:
       return if headers[CONTENT_TYPE].present?
 
       @content_type ||= Mime::HTML
-      @charset      ||= self.class.default_charset
+      @charset      ||= self.class.default_charset if !defined?(@charset) || @charset != false
 
       type = @content_type.to_s.dup
-      type << "; charset=#{@charset}" unless @sending_file
+      type << "; charset=#{@charset}" if append_charset?
 
       headers[CONTENT_TYPE] = type
     end
+
+    def append_charset?
+      !@sending_file && @charset != false
+    end
   end
 end