openapi: support content-negotiation via Accept for /workspace

[kba]

cf. OCR-D/zenhub#103

Depending on the Accept HTTP header, return either the JSON description of the workspace or the workspace itself.

[joschrew]

It looks reasonable to me what you did here but I would suggest thinking about skipping the additional content types for put and delete. It is logical to me to offer that but usually there should be no need to download before deleting or updating. Except getting and downloading in one step. But if you program a client like that it is 1st unclear and 2nd not restfull in my eyes. And users could fetch the workspace before deleting or updating (and deleting with it) by themselfs. This way the API stays easier.
Additionally I would prefer to change the description of the get-request to something like: "Return Workspace or download OCRD-ZIP " to make it clearer what can be done. To non openapi-experts like me it could be easyier to get what can be done.
But I would go with the initial changes you made as well, so feel free to completely/partly skip my suggestions.

ps: I don't know if/how I can add my change-suggestions to this PR as just a suggestion and approve anyway so I skipped that.

[kba]

ps: I don't know if/how I can add my change-suggestions to this PR as just a suggestion and approve anyway so I skipped that.

Feel free to change accordingly. For DELETE it really makes little sense, for PUT it might be, but I really don't care either way. In reality, I almost never inspect the response to POST/PUT/DELETE except for error messages.

You can directly append the PR by checking out the openapi-workspace-zip branch:

git fetch origin
git checkout -t origin/openapi-workspace-zip
# do changes, commit and push