Change the POST /v2/buildpacks/<guid>/bits to a PUT to match the app bits upload and since semantically we are replacing not adding the buildpack bits.
send nil download URI if local blob doesn't exist

This is one half of the fix for DEA staging on deployments without a
remote blobstore. The other half is in the DEA, for it to cope with
failed downloads properly. Previously it would try to extract an empty
file after failing to download.

[#58571584]
Add the curl command to the api docs