Fix ES plugin by inserting maps as strings

PR for #2549.

Note this is an ugly fix.

The final fix should be something like the one mentioned in https://github.com/bitshares/bitshares-core/pull/1396#issuecomment-432725835 :

https://www.elastic.co/guide/en/elasticsearch/reference/current/array.html :

...however, all values in the array must be of the same datatype...

That sucks. I'm surprised this doesn't bite us more often, since all static_variants serialize like this IIRC. Then again, static variant serialization sucks even more. :-/

Ok for now, but we should change this in the next release IMO. E. g.
[ tag_no, { object:data }] -> { _type: type_name, object:data }

bitshares / bitshares-core

Fix ES plugin by inserting maps as strings #2553