Do not try to call ExceptionHandler#call_exception_with_status_code on a closed response. Fixes #514

spec/exception_handler_spec.cr

@@ -102,4 +102,14 @@ describe "Kemal::ExceptionHandler" do
     response.headers["Content-Type"].should eq "application/json"
     response.body.should eq "Rendered error with 500"
   end
+
+  it "does not do anything on a closed io" do
+    get "/" do |env|
+      halt env, status_code: 404
+    end
+
+    request = HTTP::Request.new("GET", "/")
+    client_response = call_request_on_app(request)
+    client_response.status_code.should eq 404
+  end
 end

src/kemal/exception_handler.cr

@@ -18,6 +18,7 @@ module Kemal
     end
 
     private def call_exception_with_status_code(context : HTTP::Server::Context, exception : Exception, status_code : Int32)
+      return if context.response.closed?
       if !Kemal.config.error_handlers.empty? && Kemal.config.error_handlers.has_key?(status_code)
         context.response.content_type = "text/html" unless context.response.headers.has_key?("Content-Type")
         context.response.status_code = status_code