Basic auth disabled if username or password not set

[rojopolis]

The check for basic auth verifies that both username and password are set, however at least one API requires username, but no password.
For example the API for cronitor: https://cronitor.io/docs/api

Authentication to the API is performed via [HTTP Basic Auth](http://en.wikipedia.org/wiki/Basic_access_authentication). 
Provide your API key as the basic auth username value. You do not need to provide a password.

In my provider configuration I set a username, but no password and the result was that no auth was sent at all. If both are required to send auth, then it seems like it should be an error to set one but not both.

It's easy enough to work around for this particular case because the password can be set to any value and is ignored by the API. I just thought I'd open an issue in case anyone else runs into it.

https://github.com/Mastercard/terraform-provider-restapi/blob/master/restapi/api_client.go#L298-L300

[DRuggeri]

Thanks for the heads up, @rojopolis. That's an interesting use case - I haven't come across such a thing before.

It's good that you can apply any password - which would make sense because Basic Auth requires username:password base64 encoded. It's a little messy, but I think the same could be accomplished by setting the header manually if the API strictly required no password.

I'll close this one for now as something to watch as others report issues.