Add REQUEST_PATH on parse error message (#1831)

```
2019-07-01 21:31:08 +0200: HTTP parse error, malformed request (127.0.0.1/plop): #<Puma::HttpParserError: HTTP element QUERY_STRING is longer than the (1024 * 10) allowed length (was 20481)>
```

ref #1768

lib/puma/events.rb

@@ -93,7 +93,10 @@ def format(str)
     # parsing exception.
     #
     def parse_error(server, env, error)
-      @stderr.puts "#{Time.now}: HTTP parse error, malformed request (#{env[HTTP_X_FORWARDED_FOR] || env[REMOTE_ADDR]}): #{error.inspect}\n---\n"
+      @stderr.puts "#{Time.now}: HTTP parse error, malformed request " \
+        "(#{env[HTTP_X_FORWARDED_FOR] || env[REMOTE_ADDR]}#{env[REQUEST_PATH]}): " \
+        "#{error.inspect}" \
+        "\n---\n"
     end
 
     # An SSL error has occurred.

test/test_events.rb

@@ -164,4 +164,25 @@ def test_custom_log_formatter
 
     assert_equal "-> ready", out
   end
+
+  def test_parse_error
+    port = 0
+    host = "127.0.0.1"
+    app = proc { |env| [200, {"Content-Type" => "plain/text"}, ["hello\n"]] }
+    events = Puma::Events.strings
+    server = Puma::Server.new app, events
+
+    server.add_tcp_listener host, port
+    server.run
+
+    sock = TCPSocket.new host, server.connected_port
+    path = "/"
+    params = "a"*1024*10
+
+    sock << "GET #{path}?a=#{params} HTTP/1.1\r\nConnection: close\r\n\r\n"
+    sock.read
+    sleep 0.1 # important so that the previous data is sent as a packet
+    assert_match %r!HTTP parse error, malformed request \(#{path}\)!, events.stderr.string
+    server.stop(true)
+  end
 end