lib: fix implicit seqof encoding

Following schema encoding is broken:

[0] IMPLICIT SEQOF Attribute

Implicit tag is prepended to wrapping SEQ and to every element
inside it.

Implicit state should be reseted for SEQOF children
Fix default values encoding for der

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.