Evaluate changes to UCAN bearer token spec

[jsantell]

It looks like the spec recently changed for 0.3.0 where instead of multiple ucan headers, a single ucans header is used instead, with comma-separated jwts e.g. ucans: (<ucan-jwt>,)+ <ucan-jwt>

https://github.com/ucan-wg/ucan-http-bearer-token