Object IDs used in requests should be lowercase

I've implemented preliminary support for GVFS operations in Atlassian Bitbucket Server (note: not Bitbucket Cloud, e.g. bitbucket.org). In doing so, one of the things I've noticed is that GET /gvfs/objects/<sha> requests, as well as application/x-gvfs-loose-objects requests to POST /gvfs/objects, use uppercase object IDs. (Interestingly, application/x-git-packfile requests to POST /gvfs/objects use lowercase object IDs; I'm assuming that's because it drew the values from the ref advertisement, which uses lowercase values.)

When running on Windows this detail doesn't matter, since the filesystem is case-insensitive. And, naturally, I can lowercase the IDs on the server before processing (and I do so). But for the protocol, this is a nonsequitor choice. By convention, if not by absolute requirement, git always writes object IDs in lowercase hex, both on disk and in its wire protocol. For simplicity when implementing server-side processing for this, the GVFS client should also use lowercase object IDs.

Trace GVFS client handling for GET /gvfs/objects/ and loose-object POST /gvfs/objects requests, comparing it with packfile POST handling. Verify generated object IDs are lowercase hexadecimal while packfile behavior remains unchanged, using protocol or request tests if available.