Describe the Bug

Does not throw if event doesn't have a specversion attribute, and fallbacks to 1.0. The spec says that the specversion attribute is mandatory, so I think this library should fail when it's missing, instead of "magically" falling back.

Steps to Reproduce

1. Receiver.accept({ 'Content-Type': 'application/cloudevents+json' }, { id: 'foo', source: 'foo', type: 'foo' });
2. Prints console.error saying it will fallback to 1.0
3. Validation passes

Expected Behavior
It should throw, since the specversion attribute is missing