Allow forcing Content-Transfer-Encoding when encoding a Part

[mikecardwell]

Part.Encode forces base64 for any non-text content (application/*). This adds an unnecessary base64 encoding to content that the builder of the email may know to already be 7bit-safe: for example PGP/MIME and S/MIME armor. I think we should be able to override this when building an email ourselves. We could add an exported Part.ContentTransferEncoding string. If it is empty, it preserves todays current behaviour, but if it contains one of "7bit", "8bit", "base64" or "quoted-printable", we could force that encoding? Unrecognised values would fall back to auto. Happy to submit a small PR to achieve this if you agree with this and the suggested public interface?

[jhillyerd]

We should already be smart about choosing the content encoding via

enmime/encode.go

Line 306 in e5f73ef

func (p *Part) selectTransferEncoding(content []byte, quoteLineBreaks bool) transferEncoding {

-- but I haven't stepped through the entire encoding pipeline to see if something skips that.

[mikecardwell]

I think I've found a workaround:

p := enmime.NewPart("application/octet-stream")
p.Content = pgpArmor

This creates an application/octet-stream part with base64 encoding. However this leaves it as 7bit:

p := enmime.NewPart("")
p.Header.Set("Content-Type", "application/octet-stream")
p.Content = pgpArmor

With the second block of code, TextContent() returns true, so selectTransferEncoding runs and picks 7bit.