A header line with leading whitespaces is not valid in HTTP as per
RFC7230. This change ignores these invalid lines in ReadMIMEHeader.

Updates #22464

Change-Id: Iff9f00380d28a9617a55ff7888a76fba82001402
Reviewed-on: https://go-review.googlesource.com/75350Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>