node: use context-specific class for explicit

Explicit tag should be coded as 0xA0 | tag.

Signed-off-by: Fedor Indutny <fedor@indutny.com>
0.3.1
node: propagate implicit tag to uses

fix #7
0.3.2
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.