mime/multipart: don't strip leading space/tab in quoted-printable decoding

Late bug fix, but this is arguably a regression from Go 1.0,
since we added this transparent decoding since then. Without
this fix, Go 1.0 users could decode this correctly, but Go 1.1
users would not be able to.

The newly added test is from the RFC itself.

The updated tests had the wrong "want" values before. They
were there to test \r\n vs \n equivalence (which is
unchanged), not leading whitespace.

The skipWhite decoder struct field was added in the battles of
Issue 4771 in revision b3bb265bfecf. It was just a wrong
strategy, from an earlier round of attempts in
https://codereview.appspot.com/7300092/

Update issue 4771
Fixes issue 5295

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/8536045