Implement the correct method in the server_connection

[sacovo]

The first implementation used upgrade, which was then changed to upgrade_protocol, but the change wasn't reflected in the implementation of the session used by the actual http server.

[SeanTAllen]

@mfelsche @sacovo what is the bug that this fixes?

[ponylang-main]

Hi @sacovo,

The changelog - fixed label was added to this pull request; all PRs with a changelog label need to have release notes included as part of the PR. If you haven't added release notes already, please do.

Release notes are added by creating a uniquely named file in the .release-notes directory. We suggest you call the file 78.md to match the number of this pull request.

The basic format of the release notes (using markdown) should be:

## Title

End user description of changes, why it's important,
problems it solves etc.

If a breaking change, make sure to include 1 or more
examples what code would look like prior to this change
and how to update it to work after this change.

Thanks.

[SeanTAllen]

So, there's a default method on the Session trait. Which means that the incorrectly named upgrade instead of upgrade_protocol on _server_connection isn't caught. That's the fix here and should be indicated as such in the release notes.

[mfelsche]

Thanks for catching this @sacovo !

If this gets some release notes, this is good to go.