Repository Contents API doesn't support Base64 content directly

[shiftkey]

As raised in #925, we're making the assumption here that you're interested in publishing a UTF-8 string as your content when you create or update a file. That's not entirely true.

As raised in #925 there's a workaround which is to use the Git Data API - see sample here #925 (comment) - but I'll leave this open to indicate an area where we don't currently support it.

[laedit]

If that's ok I want to contribute on this one.

About how to do it, I think that the simplest is to add an EncodingType Encoding property, potentially add the corresponding constructors and remove the SerializeAsBase64 attribute on Content property.

And if you want to avoid any breaking change on this, deal with the base64 in the existing constructors.

What do you think?

[shiftkey]

@laedit go for it - there might be some things to catch when serializing/deserialzing but I'm happy to review things and ensure a smooth transition