Add `Date` header to HTTP responses (#676)

spec/init_handler_spec.cr

@@ -11,6 +11,18 @@ describe "Kemal::InitHandler" do
     context.response.headers["Content-Type"].should eq "text/html"
   end
 
+  it "initializes context with Date header" do
+    request = HTTP::Request.new("GET", "/")
+    io = IO::Memory.new
+    response = HTTP::Server::Response.new(io)
+    context = HTTP::Server::Context.new(request, response)
+    Kemal::InitHandler::INSTANCE.next = ->(_context : HTTP::Server::Context) {}
+    Kemal::InitHandler::INSTANCE.call(context)
+    date = context.response.headers["Date"]?.should_not be_nil
+    date = HTTP.parse_time(date).should_not be_nil
+    date.should be_close(Time.utc, 1.second)
+  end
+
   it "initializes context with X-Powered-By: Kemal" do
     request = HTTP::Request.new("GET", "/")
     io = IO::Memory.new

src/kemal/init_handler.cr

@@ -1,3 +1,5 @@
+require "http"
+
 module Kemal
   # Initializes the context with default values, such as
   # *Content-Type* or *X-Powered-By* headers.
@@ -9,6 +11,7 @@ module Kemal
     def call(context : HTTP::Server::Context)
       context.response.headers.add "X-Powered-By", "Kemal" if Kemal.config.powered_by_header?
       context.response.content_type = "text/html" unless context.response.headers.has_key?("Content-Type")
+      context.response.headers.add "Date", HTTP.format_time(Time.utc)
       call_next context
     end
   end