TinyMCE overwrite my valid "required" attribute to an invalid one

TinyMCE overwrite my valid required attribute to an invalid one.

I use TinyMCEv 4.7.4. I know it's old a little bit, but it is not possible to replace it, becasue of dependencies.

This is my config:

{
  mode: "exact",
  elements : "elm1",
  extended_valid_elements: "hr[*],font[*],span[*],img[class,*],iframe[*],div[*],embed[*],script[*],a[*]",
}

I type this:

<input name="email" type="text" required />

And the editor change to this:

<input name="email" type="text" required="" />

And now the browsers don't handle the input as required one.

How can I solve this issue? Is there any specific option to tell TinyMCE "don't touch the required attribute" ?