Use `default_type` instead of `add_header Content-Type`

The latter can cause duplicate Content-Type headers. Thanks to strcat
for notifying me of the issue.

roles/archweb/templates/maintenance-nginx.d.conf.j2

@@ -92,12 +92,12 @@ server {
     error_page 503 /503.html;
 
     location /.well-known/matrix/server {
-        add_header Content-Type application/json;
+        default_type application/json;
         return 200 '{"m.server": "{{ matrix_domain }}:443"}';
     }
 
     location /.well-known/matrix/client {
-        add_header Content-Type application/json;
+        default_type application/json;
         add_header Access-Control-Allow-Origin *;
         return 200 '{"m.homeserver": {"base_url": "https://{{ matrix_domain }}"}, "m.identity_server": {"base_url": "https://matrix.org"} }';
     }

roles/archweb/templates/nginx.d.conf.j2

@@ -114,12 +114,12 @@ server {
     ssl_trusted_certificate /etc/letsencrypt/live/{{ archweb_domain }}/chain.pem;
 
     location /.well-known/matrix/server {
-        add_header Content-Type application/json;
+        default_type application/json;
         return 200 '{"m.server": "{{ matrix_domain }}:443"}';
     }
 
     location /.well-known/matrix/client {
-        add_header Content-Type application/json;
+        default_type application/json;
         add_header Access-Control-Allow-Origin *;
         return 200 '{"m.homeserver": {"base_url": "https://{{ matrix_domain }}"}, "m.identity_server": {"base_url": "https://matrix.org"} }';
     }

roles/aurweb/templates/nginx.d.conf.j2

@@ -66,7 +66,7 @@ server {
     }
 
     location ~ \.gz$ {
-        add_header Content-Type text/plain;
+        default_type text/plain;
         add_header Content-Encoding gzip;
         expires 5m;
     }

roles/matrix/templates/nginx.d.conf.j2

@@ -61,13 +61,13 @@ server {
 
 {% endfor %}
         location = /metrics {
-            add_header Content-Type text/plain;
+            default_type text/plain;
             return 200 "Available endpoints:\n{% for ep in matrix_metrics_endpoints %}  /metrics/{{ ep.name }}\n{% endfor %}";
         }
     }
 
     location = / {
-        add_header Content-Type text/plain;
+        default_type text/plain;
         return 200 "Nothing to see here.";
     }