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.

Fixes https://github.com/owncloud/core/issues/10383

Please review @DeepDiver1975 @icewind1991 @schiesbn @guruz