Properly encode IMPLICIT tag

Add tests for IMPLICIT and EXPLICIT tags with submodel and fix broken code.
Well, the thing is - implicit tag with submodel can't be handled with `_encodeComposite` at this line.

This just leads to exception because `content` variable is not set when we have `use` (see https://github.com/indutny/asn1.js/blob/master/lib/asn1/base/node.js#L491).

Instead whole encoding is done at line 492