Setting password without username should raise an error

[jsvd]

Starting the plugin with input { http { password => 'hello'} } will not enable auth and also raise no warning or error that both username and password are needed to setup basic auth.

replaces #63

[knaggit]

I just stumble over this issue here and would appreciate change this behaviour. It even would be nice for things like webhooks which just require a secret (regarding to GitHub webhooks), to configure just a password. Above that I totally agree with the bug description.