Fix Nuget nuspec files UTF-16 encoding issue

[sverdlov93]

• [ ] All tests passed. If this feature is not already covered by the tests, I added new tests.
• [ ] All static analysis checks passed.
• [ ] This pull request is on the dev branch.

• [ ] I used gofmt for formatting the code before submitting the pull request.

  Sometimes the nuspec file is wrongly encoded in utf-16, the actual encoding is utf-8 but the xml header has enocding="utf-16" key. xml.Unmarshal doesn't support utf-16 encoding, so we need to convert the header to utf-8.

should fix https://github.com/jfrog/jfrog-cli/issues/840 and https://github.com/jfrog/jfrog-azure-devops-extension/issues/464