new_options should be optional in asset_update_operation

_From @theoreticalbts on February 15, 2016 15:28_

`new_options` should be optional allowing `asset_update_operation` with semantics of "don't change the options" (but maybe change the issuer).

It's going to be hard to implement this change without breaking protocol compatibility.

_Copied from original issue: cryptonomex/graphene#580_

Update: https://github.com/bitshares/bsips/issues/159 will cover this.

Start with the linked BSIPS issue 159, which is identified as covering this change, then inspect the protocol definition and handling of asset_update_operation. Done means new_options can be omitted without changing existing options, issuer changes remain possible, and protocol compatibility is addressed.

Written by the indexing model from the issue text.