HeadRequestHandler: run GET handler and don't return the body (#655)

spec/head_request_handler_spec.cr

@@ -0,0 +1,37 @@
+require "./spec_helper"
+
+describe "Kemal::HeadRequestHandler" do
+  it "implicitly handles GET endpoints, with Content-Length header" do
+    get "/" do
+      "hello"
+    end
+    request = HTTP::Request.new("HEAD", "/")
+    client_response = call_request_on_app(request)
+    client_response.body.should eq("")
+    client_response.headers["Content-Length"].should eq("5")
+  end
+
+  it "prefers explicit HEAD endpoint if specified" do
+    Kemal::RouteHandler::INSTANCE.add_route("HEAD", "/") { "hello" }
+    get "/" do
+      raise "shouldn't be called!"
+    end
+    request = HTTP::Request.new("HEAD", "/")
+    client_response = call_request_on_app(request)
+    client_response.body.should eq("")
+    client_response.headers["Content-Length"].should eq("5")
+  end
+
+  it "gives compressed Content-Length when gzip enabled" do
+    gzip true
+    get "/" do
+      "hello"
+    end
+    headers = HTTP::Headers{"Accept-Encoding" => "gzip"}
+    request = HTTP::Request.new("HEAD", "/", headers)
+    client_response = call_request_on_app(request)
+    client_response.body.should eq("")
+    client_response.headers["Content-Encoding"].should eq("gzip")
+    client_response.headers["Content-Length"].should eq("25")
+  end
+end