[SON for Hive] - Fix asset_symbol_type serialization

Comparing Hive wallet and SON for Hive transaction processing, we do have exactly the same string representation of the transactions:

Hive
{"ref_block_num":42214,"ref_block_prefix":2560592527,"expiration":"2021-04-26T10:38:51","operations":[{"type":"transfer_operation","value":{"from":"sonaccount01","to":"account05","amount":{"amount":"1000","precision":3,"nai":"@@000000021"},"memo":""}}],"extensions":[],"signatures":[]}

SON
{"ref_block_num":42214,"ref_block_prefix":2560592527,"expiration":"2021-04-26T10:38:51","operations":[{"type":"transfer_operation","value":{"from":"sonaccount01","to":"account05","amount":{"amount":"1000","precision":3,"nai":"@@000000021"},"memo":""}}],"extensions":[],"signatures":[]}

When we serialize transaction for signing, we get different binary representations

Hive
E6A48F8A9F983B98866001020C736F6E6163636F756E743031096163636F756E743035E8030000000000000354455354530000000000

SON
E6A48F8A9F983B98866001020C736F6E6163636F756E743031096163636F756E743035E8030000000000002320BCBE000000

Dissected, this is the binary layout

Hive:
E6A4 8F8A9F98 3B98866001 02 0C736F6E6163636F756E743031 09 6163636F756E743035 E803000000000000 0354455354530000 000000

E6A4                       ref_block_num
8F8A9F98                   ref_block_prefix
3B98866001                 expiration
02                         operation identifier (transfer)
0C736F6E6163636F756E743031 from (string of length 13 (0C), this is actually sonaccount01 in hex)
096163636F756E743035       to (string of length 9 (09), this is actually account05 in hex)
E803000000000000           amount.amount, 1000 in hex, 64 bit
0354455354530000           this is asset identifier, 03 - precision and TESTS in hex
000000                     ???

SON:
E6A4 8F8A9F98 3B98866001 02 0C736F6E6163636F756E743031 09 6163636F756E743035 E803000000000000 2320BCBE000000

E6A4                       ref_block_num
8F8A9F98                   ref_block_prefix
3B98866001                 expiration
02                         operation identifier (transfer)
0C736F6E6163636F756E743031 from (string of length 13 (0C), this is actually sonaccount01 in hex)
096163636F756E743035       to (string of length 9 (09), this is actually account05 in hex)
E803000000000000           amount.amount, 1000 in hex, 64 bit
2320BCBE000000             incorrectly serialized asset_symbol_type

There are pack/unpack functions here: !13 (diffs)

which should handle asset_symbol_type serialization, but for some reason fc lib does not picks it for serialization.