Der default values

DER encoder should not include values that are equual to defaults.

Ref http://luca.ntop.org/Teaching/Appunti/asn1.html:

    DER encoding. Constructed. Contents octets are the same as the BER
    encoding, except that if the value of a component with the DEFAULT
    qualifier is the default value, the encoding of that component is not
    included in the contents octets.
In other words, this will work for primitives like strings, numbers, but won't work for complex data in general. Perhaps we should compare output DER encoding here instead?