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.
Perhaps this function could just accept result as an input and return it in case of success? Is there any point of doing it here?