StringToDate doesn't parse all common RFC 3339 strings

[raehik]

Redirected from Hugo issue gohugoio/hugo#5340 .

2006-01-02T15:04:05-0700 is a valid RFC 3339 date which is not successfully parsed by cast. Obviously we shouldn't be expected to parse every single date format on the planet, or even all the various RFC 3339 formats. strftime however can only output its timezone in ±hhmm format, so the omitted colon form is relatively common.

It's a trivial change, so I may make an associated PR.