Avoid calling Headers#env_name when static header

`env_name` has to transform given strings into the `HTTP_` type headers
that are put in the Rack env. This involves at least one String
allocation and a few additional transformations performed on the string.

This commit avoids the extra allocations/transformations by precomputing
some request header strings so that `env_name` can be avoided. The
places changed here show up in a basic controller request, but there may
be other places that could be changed as well.

actionpack/lib/action_dispatch/http/request.rb

@@ -55,6 +55,8 @@ def #{env.delete_prefix("HTTP_").downcase}  # def accept_charset
       METHOD
     end
 
+    TRANSFER_ENCODING = "HTTP_TRANSFER_ENCODING" # :nodoc:
+
     def self.empty
       new({})
     end
@@ -282,7 +284,7 @@ def media_type
 
     # Returns the content length of the request as an integer.
     def content_length
-      return raw_post.bytesize if headers.key?("Transfer-Encoding")
+      return raw_post.bytesize if has_header?(TRANSFER_ENCODING)
       super.to_i
     end
 
@@ -467,7 +469,7 @@ def default_session
 
       def read_body_stream
         reset_stream(body_stream) do
-          if headers.key?("Transfer-Encoding")
+          if has_header?(TRANSFER_ENCODING)
             body_stream.read # Read body stream until EOF if "Transfer-Encoding" is present
           else
             body_stream.read(content_length)

actionpack/lib/action_dispatch/middleware/request_id.rb

@@ -25,11 +25,12 @@ class RequestId
     def initialize(app, header:)
       @app = app
       @header = header
+      @env_header = "HTTP_#{header.upcase.tr("-", "_")}"
     end
 
     def call(env)
       req = ActionDispatch::Request.new env
-      req.request_id = make_request_id(req.headers[@header])
+      req.request_id = make_request_id(req.get_header(@env_header))
       @app.call(env).tap { |_status, headers, _body| headers[@header] = req.request_id }
     end