add SetBoundary(boundary string) method

[vodolaz095]

This method is used to manually set mime boundary string. If we don't call this method, the default behaviour is used - aka boundary is generated by https://godoc.org/mime/multipart package.
Also i have added unit tests to ensure it works as expected. And all old unit tests are passing.

Reason i want this method to be present, is because i have to deal with legacy report processing java application, and it hangs sometimes, when i send messages generated by your package.
As far as i understand how this java dinosaur works, it throws errors because boundary generated is 30 bytes long.

https://golang.org/src/mime/multipart/writer.go#L78

func randomBoundary() string {

	var buf [30]byte

	_, err := io.ReadFull(rand.Reader, buf[:])

	if err != nil {

		panic(err)

	}

	return fmt.Sprintf("%x", buf[:])

}

And it seems too big for java dinosaur code. Or maybe badly written regex in it... hard to say.
But if i could set boundary manually, you will make my day!
thanks

[pedromorgan]

Please check #108

[vodolaz095]

and what? you propose to remake pull request to http://github.com/go-mail/mail?

[pedromorgan]

I am just pointing out that any PR's here wont go anywhere atmo, unfortunately see #104. There are a few forks, but the one above seems to have merged some of the outstanding PR's from this repos.

[ivy]

Correct me if I'm wrong, it doesn't look like there's any need for this assignment unless we decide to use it on L87 in place of mw.Boundary().

Instead, this could be expressed as:

if boundary != "" {
	mw.SetBoundary(boundary)
}

[vodolaz095]

yes, thanks

[ivy]

@vodolaz095 Your change looks good to me. I'd definitely like to merge it into go-mail/mail which is actively maintained.