Prefer lowercase headers in ActionDispatch::Static.

`ActionDispatch::Static` uses mixed-case headers and merges them with
lower case headers. This produces duplicate headers. Prefer lowercase
headers to avoid this situation.

Author: ioquatix

actionpack/lib/action_dispatch/middleware/static.rb

@@ -31,7 +31,7 @@ def call(env)
   #
   # Precompressed versions of these files are checked first. Brotli (.br)
   # and gzip (.gz) files are supported. If +path+.br exists, this
-  # endpoint returns that file with a <tt>Content-Encoding: br</tt> header.
+  # endpoint returns that file with a <tt>content-encoding: br</tt> header.
   #
   # If no matching file is found, this endpoint responds <tt>404 Not Found</tt>.
   #
@@ -76,7 +76,7 @@ def serve(request, filepath, content_headers)
           request.path_info, ::Rack::Utils.escape_path(filepath).b
 
         @file_server.call(request.env).tap do |status, headers, body|
-          # Omit Content-Encoding/Type/etc headers for 304 Not Modified
+          # Omit content-encoding/type/etc headers for 304 Not Modified
           if status != 304
             headers.update(content_headers)
           end
@@ -104,7 +104,7 @@ def find_file(path_info, accept_encoding:)
       end
 
       def try_files(filepath, content_type, accept_encoding:)
-        headers = { "Content-Type" => content_type }
+        headers = { "content-type" => content_type }
 
         if compressible? content_type
           try_precompressed_files filepath, headers, accept_encoding: accept_encoding
@@ -124,10 +124,10 @@ def try_precompressed_files(filepath, headers, accept_encoding:)
             if content_encoding == "identity"
               return precompressed_filepath, headers
             else
-              headers["Vary"] = "Accept-Encoding"
+              headers["vary"] = "accept-encoding"
 
               if accept_encoding.any? { |enc, _| /\b#{content_encoding}\b/i.match?(enc) }
-                headers["Content-Encoding"] = content_encoding
+                headers["content-encoding"] = content_encoding
                 return precompressed_filepath, headers
               end
             end