Sinject::Container#register allows key parameter to be nil #14

Sinject::Container#register will check that a key parameter has been provided and raise an error if it is absent. However, it does not check that the key is non-nil.

Steps to reproduce:

require 'sinject'
container = Sinject::Container.new(false)
container.register(key: nil, class: Object)

Expected:

Sinject::DependencyRegistrationKeyNotSpecifiedException or similar to be raised.

Actual:

• Nothing is raised.

• container.get(nil) returns an instance of Object.