Feature request: ordered headers

[iredmail]

Dear @jhillyerd,

Mail headers stored in header attribute is unordered (the underlying type of *textproto.MIMEHeader is map[string][]string):

enmime/envelope.go

Line 28 in 5cd8001

header *textproto.MIMEHeader // Header from original message

I wonder whether you're willing to accept PR to make it an ordered map instead. Since header is an internal attribute, i expect no breaking changes will be introduced.

Ordered headers is very useful when you just modified part of mail body and want to keep the order.

[jhillyerd]

This sounds good to me.