Converted *.nuspec files to UTF-8

Another tiny step to get rid of the legacy Windows-1252 encoding. :grinning: 

Keep in mind that GitHub has problems showing diffs when a file is converted between character encodings (Windows-1252 to UTF-8 in this case) and will show the UTF-8 characters as if they were encoded as Windows-1252, e.g. `á` to `Ã¡`

But the conversion to UTF-8 in this commit is correct. Click on the “View” button of a file and you will see that the characters are the correct and valid UTF-8.