Prevent logging unsafe header error
Prevent logging unsafe header error

Backend that don't support TUS will also not expose TUS headers through
CORS. To prevent the uncatchable "refused to get unsafe header", this
commit adds a workaround with a preliminary check using
"getAllResponseHeaders" to find out if the header is exposed by CORS
before reading it.

noveens/js-owncloud-client,noveens/js-owncloud-client,noveens/js-owncloud-client