Handle connection key being empty to support Google Colaboratory

[shadaj]

When running kernels on Google Colaboratory, the key value in connection.json is empty to indicate that authentication and signature checking should be disabled (noted in the official docs). This PR updates ZeromqSocketImpl to handle the situation where key is empty by using an empty string as the signature when sending a message and skipping the signature check when receiving a message. With these changes, Almond works perfectly inside Colaboratory.

[alexarchambault]

Thanks @shadaj!

Maybe we could add a test case for it in ZeromqSocketTests, to prevent regressions. I guess a test like the current simple one, but with an empty key, should be enough.

[shadaj]

@alexarchambault just added in a unit test!

[alexarchambault]

Thanks, merging!

[laughedelic]

How can I enable Almond in Colab?

[alexarchambault]

@laughedelic I've been installing it by running this notebook. Then importing a notebook (e.g. from the example repo) seems to work.

[alexarchambault]

Don't know if @shadaj does it this way.

[laughedelic]

Cool! Thanks for sharing! I didn't know that I can import notebooks.

[shadaj]

Working on a blog post coming soon :)

[alexarchambault]

Looking forward to it!

I mentioned Colaboratory in the release notes for 0.3.1. They are on their way to the blog (once the CI has pushed them there).