fix(model): drop non-special properties starting with '_'

Properties which start with a '_' are special for PouchDB (and maybe
CouchDB as well?), so persisting them into the cache would cause
errors.
refactor(model): rename fixDoubleBackslashes() to fixDoubleSlashes()

'/' is a slash, not a backslash!
refactor(model): make toURL() return the base prepended
test(modelProvider): ensure http:// is kept as base URL
test(model): ensure empty passwords are properly used in .auth() method
test(model): test usage of .list() in elements
refactor(model): comment on rejection of empty cached collections