bug: extra attributes are not validated against the spec

[richardvflux]

When extra attributes are added to the CloudEvent, they are not being checked for compliance.

The spec says:

CloudEvents attribute names MUST consist of lower-case letters ('a' to 'z') or digits ('0' to '9') from the ASCII character set. Attribute names SHOULD be descriptive and terse and SHOULD NOT exceed 20 characters in length.

But currently anything can be added and it is accepted. It should throw an AttributeException as listed in the errors.rb file.