net/textproto: mime header Get expects canonical header form

[eaigner]

If a textproto.MIMEHeader is initialized from a source where header keys are not
canonical, Get won't work on those keys.

  http://play.golang.org/p/_yeMZaJdZv

I suggest comparing the keys using strings.EqualFold instead. It is quite common that
headers are not in their canonical form.

[eaigner]

Comment 1:

Maybe a method to normalize the header would be more efficient, than running through all
the keys on Get.
   // Canonical returns the header with all keys converted to their canonical form.
   func (h MIMEHeader) Canonical() MIMEHeader

[gopherbot]

Comment 2 by islandberry23:

Use http://golang.org/pkg/net/textproto/#CanonicalMIMEHeaderKey to canonicalize the keys
when constructing the map.

[robpike]

Comment 3:

Status changed to WorkingAsIntended.