Support charset

[ffes]

This adds support for the charset property. It converts the current document to the right charset before saving it.

Fixes #41

[ffes]

The failure of the CI seems to be the same issue as #31

[xuhdev]

I see the error message:

The server did not return the file size. The URL might point to dynamic content. The Content-Length header is not available in the server's HTTP reply.

Maybe the -Dynamic flag should be turned on? https://learn.microsoft.com/en-us/powershell/module/bitstransfer/start-bitstransfer?view=windowsserver2022-ps

[ffes]

Maybe the -Dynamic flag should be turned on?

That helped for GitHub Actions, thanks for the tip!

But for appveyor it doesn't work. It's PowerShell doesn't seem to recognise the -Dynamic flag

[xuhdev]

Not sure if we should eventually drop appveyor for this repo. Let's merge this first.