Avoid XML errors

libs/web/luasrc/http.lua

@@ -192,14 +192,16 @@ end
 --- Set the mime type of following content data.
 -- @param mime	Mimetype of following content
 function prepare_content(mime)
-	if mime == "application/xhtml+xml" then
-		if not getenv("HTTP_ACCEPT") or
-		  not getenv("HTTP_ACCEPT"):find("application/xhtml+xml", nil, true) then
-			mime = "text/html; charset=UTF-8"
+	if not context.headers or not context.headers["content-type"] then
+		if mime == "application/xhtml+xml" then
+			if not getenv("HTTP_ACCEPT") or
+			  not getenv("HTTP_ACCEPT"):find("application/xhtml+xml", nil, true) then
+				mime = "text/html; charset=UTF-8"
+			end
+			header("Vary", "Accept")
 		end
-		header("Vary", "Accept")
+		header("Content-Type", mime)
 	end
-	header("Content-Type", mime)
 end
 
 --- Get the RAW HTTP input source

modules/admin-full/luasrc/controller/admin/system.lua

@@ -213,6 +213,7 @@ function action_upgrade()
 		end
 	end
 
+	luci.http.prepare_content("text/html")
 	luci.template.render("admin_system/upgrade", {sysupgrade=plat, ret=ret, keep_avail=keep_avail})
 end
 

modules/admin-mini/luasrc/controller/mini/system.lua

@@ -110,6 +110,7 @@ function action_upgrade()
 		end
 	end
 
+	luci.http.prepare_content("text/html")
 	luci.template.render("mini/upgrade", {sysupgrade=plat, ret=ret, keep_avail=keep_avail})
 end