ctx.estimatedSize provided to _openWriteStream is incorrect on Mac OS

[qali-clio]

When implementing a custom FileSystem (as per https://github.com/OpenMarshal/npm-WebDAV-Server/wiki/Custom-File-System-%5Bv2%5D), _openWriteStream receives an OpenWriteStreamInfo that contains the size of the incoming file. On Mac OS, the size in this parameter is -1 because Finder does not provide a Content-Length header.

Instead, servers interacting with Finder should use the length from the X-Expected-Entity-Length header. Extracting the estimated size from the X-Expected-Entity-Length header with a fallback to Content-Length works on both Mac and Windows.