mailinfo: be more liberal with header whitespace

mailinfo: be more liberal with header whitespace

RFC822 and friends allow arbitrary whitespace after the colon of a
header and before the values. I.e.:

  Subject:foo
  Subject: foo
  Subject:  foo

all have the subject "foo". But mailinfo requires exactly one space.
This doesn't seem to be bothering anybody, but it is pickier than the
standard specifies. And we can easily just soak up arbitrary whitespace
there in our parser, so let's do so.

Note that the test covers both too little and too much whitespace, but
the "too much" case already works fine (because we later eat leading and
trailing whitespace from the values).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>