Allow vote and unvote in the same tx

[air1one]

Allow vote/unvote in the same tx, like

{ asset: { votes: [
 "+0366da7ed85cc1e73a13fafb8007d0609d92239355847343cc7b12d85a65d25502",
"-03153c994e5306b1fbba9bb533f22871e12e4c1d1d3960d1eeef385ab143b258b4"
] } }

This way we could be backward-compatible (no change needed in ser/deserialize methods, just schema and handlers update).