Return whole file if range request cannot be granted due to encryption

Whenenver range headers are set and encryption is enabled, it is not
possible to automatically fseek() to the proper position.

To avoid returning corrupt/invalid data or causing a decryption error,
the range headers are stripped so that the SabreDAV code in httpGet()
returns the whole file.

Backport of cc8c1d8e0776bda7f4c7c28e11984144a4861df3 from master