nextcloud: Add Content-Length headers for uploading

[provokateurin]

Fixes #332

@muelleel please test.

[muelleel]

Okay, now this gets tricky,

I'm depending on PR #333, using this PR's commit bf3b0b1 is resulting in the known Error on line 15, column 11: Invalid description in the "nextcloud" pubspec on the "dynamite_runtime" dependency: "../dynamite/dynamite_runtime" is a relative path, but this isn't a local pubspec.

Can I somehow "combine" these two commits?
Sorry, at this point my git knowledge ends :(

[provokateurin]

You can drop the other PR and do a git cherry-pick of this commit on top of your last working commit.

[muelleel]

IT WORKS :D
Uploading now works just as expected.

Thank you very much