UTF-8 for robots.txt.

snippets/robots-allow-all.conf

 location = /robots.txt {
-    add_header Content-Type text/plain;
+    add_header Content-Type "text/plain; charset=utf-8";
     return 200 "User-agent: *\nDisallow:\n";
 }

snippets/robots-disallow-all.conf

 location = /robots.txt {
-    add_header Content-Type text/plain;
+    add_header Content-Type "text/plain; charset=utf-8";
     return 200 "User-agent: *\nDisallow: *\n";
 }