Change the default failure in parse_http_date() to NA with class Date

[shrektan]

The default value of failure argument in parse_http_date() should be NA with class Date.

Otherwise, print.response() may throw an unfriendly error.

On a Chinese-language Windows machine, I use plumber to build an REST API. The API returns the header$date in Chinese format by default, like "周三, 10 十月 2018 1:19:13 GMT".

parse_http_date() will fail to parse it correctly and fall back to the default value of `failure``.

However, the default value of failure is set to NA (the logical value). Whenever I try to print the response, R will throw error saying:

Error in prettyNum(.Internal(format(x, trim, digits, nsmall, width, 3L, : invalid 'trim' argument

I believe the cause is

httr/R/response.r

Line 35 in 976289a

cat(" Date: ", format(x$date, "%Y-%m-%d %H:%M"), "\n", sep = "")

After this PR, the result can be printed successfully now.

Response [http://127.0.0.1:8001/reLoad]
  Date: NA
  Status: 200
  Content-Type: application/json
  Size: 2 B

[hadley]

Thanks!