Allow to parse incomplete mixed multipart
Allow to parse incomplete mixed multipart

Mixed multipart without proper boundary was considered error
previously. Now it is not error but just ignoring it by assuming
it is empty. This is to solve FS#4760 too. This allows more email
parsing can be executed successfully.

Also use modified net/textproto pkg to parse any valid MIME
headers that sit within invalid headers.

cention-sany/go.enmime