append_url_extension unignored

[scaccogatto]

I found that this header variable:

append_url_extension: json

is not correclty used within Page.php rendering section.

Here is the fix!

[rhukster]

I need to confirm that this won't have the unintended result of appending the default extension when it's not wanted.

[scaccogatto]

I would like to bring focus on this again: after the last update the problem is still here.
I use this patch in a production website and there is no problem at all.

[rhukster]

I've manually merged this into Grav 1.6 branch