spec: Add content type header in blob uploads

[dmcgowan]

Add the content type header to blob upload endpoints with language about what the value should be. In the case of blobs, the content type should be ignored and this is best represented as using the application/octet-stream Content-Type. Any other types will be ignored by a registry and not be reflected when that blob is pulled.

I don't think a 415 return status is appropriate when application/octet-stream is not used. We should explicitly call that out since ignored != unsupported. Ping @rchincha

[rchincha]

So when pushing Content-Type is just an opaque label. However, not clear from your comment above on what the behavior should be a client pulls with an Accept? If this header not present, then return whatever it was pushed with. But if present and pull(Accept) != pushed(Content-Type), what happens?

[vbatts]

Isn't this more clearly laid out in #212 ?
(i'm thinking we can close this current issue)

[jonjohnsonjr]

What does this solve?