node: don't set missing keys to null

If key is missing from input it should not be set
to null in resulting object because such object
can't be but back into encoder.
0.3.3
deps: use bn.js
0.4.0
rfc3280: 0.4.0
rfc2560: 0.4.0
Properly encode IMPLICIT tag
Fix both explicit and implicit tags for submodel
rfc-3280: fix Certificate encoding
rfc3280: 0.4.1
node: fix encoding of CHOICE
0.4.1
base: plug submodels with function call

Implemented as suggested in #14

Reviewed-By: Fedor Indutny <fedor@indutny.com>
0.5.0
rfc: update peer deps
rfc: bump versions
lib: better error reporting
0.5.1
der: support `ia5str`
0.6.0