rais-server: don't marshal info JSON immediately
rais-server: don't marshal info JSON immediately

Refactors the response helpers so they're sending/receiving the
iiif.Info structure rather than raw byte slices.  We can use this to
test for unsupported size requests, and in the future this would let us
use overridden info.json data to restrict capabilities on a per-image
basis.

uoregon-libraries/rais-image-server,uoregon-libraries/rais-image-server,uoregon-libraries/rais-image-server,uoregon-libraries/rais-image-server