Let's make sure the constructor of ProfileElement subclasses set
meaningful defaults to the self.decoded member, so that the to_der()
method can actually encode it. This is required when constructing
a profile from scratch, as opposed to loading an existing one from DER.
Also, add a test to verify that the encoder passes without exception;
doesn't test the generated binary data.
Change-Id: I401bca16e58461333733877ec79102a5ae7fe410
200bf6eb8b