[test] missing 80+ tests for webdav_server

[thenil3sh]

Current library only covers very few tests for whatever we know, but again, that misses a lot of cases that code covers, but the tests don't. That counts to around 79, in total, across all modules.

Following layers are missing tests, I'll cover them one by one

• Model layer
• Storage
• Api Layer (it's barely got any test)
• Logger
• Error (zero tests)
• Integration tests (again, useless tests)