Only serve wp-supercached static files when Accept includes text/html

Attempts to fix the content negotiation caching issue #134.

docker/htaccess

@@ -45,6 +45,7 @@ RewriteCond %{REQUEST_URI} !^.*[^/]$
 RewriteCond %{REQUEST_URI} !^.*//.*$
 RewriteCond %{REQUEST_METHOD} !POST
 RewriteCond %{QUERY_STRING} ^$
+RewriteCond %{HTTP:Accept} text/html
 RewriteCond %{HTTP:Cookie} !^.*(comment_author_|wordpress_logged_in|wp-postpass_).*$
 RewriteCond %{HTTP:X-Wap-Profile} !^[a-z0-9\"]+ [NC]
 RewriteCond %{HTTP:Profile} !^[a-z0-9\"]+ [NC]