HTTP/2: Cache whether the port is default in the conn

When constructing the authority pseudo header we check if the conn's
port is the default for the conn's scheme using `URI.default_port/1`.
That corresponds to an ETS lookup into the `:elixir_config` table. It's
relatively fast to read that information but the conn's port and scheme
are static for the life of the conn, so we should determine this
information once while initiating the conn (`Mint.HTTP2.initiate/5`).
This change saves a small amount of time per request and becomes more
valuable as the conn is re-used for more requests.

lib/mint/http2.ex

@@ -171,6 +171,9 @@ defmodule Mint.HTTP2 do
     :port,
     :scheme,
 
+    # Whether the port is the default for the scheme
+    :default_port?,
+
     # Connection state (open, closed, and so on).
     :state,
 
@@ -974,6 +977,7 @@ defmodule Mint.HTTP2 do
         ) :: {:ok, t()} | {:error, Types.error()}
   def initiate(scheme, socket, hostname, port, opts) do
     transport = scheme_to_transport(scheme)
+    scheme_string = Atom.to_string(scheme)
     mode = Keyword.get(opts, :mode, :active)
     log? = Keyword.get(opts, :log, false)
     client_settings_params = Keyword.get(opts, :client_settings, [])
@@ -992,10 +996,11 @@ defmodule Mint.HTTP2 do
     conn = %__MODULE__{
       hostname: hostname,
       port: port,
+      default_port?: URI.default_port(scheme_string) == port,
       transport: scheme_to_transport(scheme),
       socket: socket,
       mode: mode,
-      scheme: Atom.to_string(scheme),
+      scheme: scheme_string,
       state: :handshaking,
       log: log?
     }
@@ -1358,15 +1363,15 @@ defmodule Mint.HTTP2 do
     if String.upcase(method) == "CONNECT" do
       [
         {":method", method},
-        {":authority", authority_pseudo_header(conn.scheme, conn.port, conn.hostname)}
+        {":authority", authority_pseudo_header(conn)}
         | headers
       ]
     else
       [
         {":method", method},
         {":path", path},
         {":scheme", conn.scheme},
-        {":authority", authority_pseudo_header(conn.scheme, conn.port, conn.hostname)}
+        {":authority", authority_pseudo_header(conn)}
         | headers
       ]
     end
@@ -1720,12 +1725,12 @@ defmodule Mint.HTTP2 do
   end
 
   # If the port is the default for the scheme, don't add it to the :authority pseudo-header
-  defp authority_pseudo_header(scheme, port, hostname) do
-    if URI.default_port(scheme) == port do
-      hostname
-    else
-      "#{hostname}:#{port}"
-    end
+  defp authority_pseudo_header(%__MODULE__{default_port?: true, hostname: hostname}) do
+    hostname
+  end
+
+  defp authority_pseudo_header(%__MODULE__{hostname: hostname, port: port}) do
+    "#{hostname}:#{port}"
   end
 
   defp join_cookie_headers(headers) do