Fix unnecessary error on failure to parse headers$Date

[stefanfritsch]

Hi,

when parsing the headers$Date in request_perform fails, e.g. because the response is not in en_US it results in an unhelpful prettyNum (or so) error.

I just split the if-else to replace the date with something valid if it failed to parse.

Ciao,
Stefan

[hadley]

Could you please give me an example of the error that this resolves? I don't understand what you mean by prettyNum

[hadley]

Oh I see this is a duplicate of #544, which explains the problem better.