kmd.sign_transaction() fails when signing_address is specified

### algosdk.kmd.sign_transaction() fails when `signing_address` arg is specified

### Expected behaviour
Transaction should be signed using the private key corresponding to the specified `signing_address`

### Actual behaviour
Exception is raised:
```
algosdk.error.KMDHTTPError: {
"error": true,
"message": "could not decode request body"
```

### Steps to reproduce issue
1. start sandbox environment
2. Run the below code to reproduce
```
from pprint import pprint
from typing import cast

from algosdk import kmd
from algosdk.transaction import PaymentTxn
from algosdk.v2client.algod import AlgodClient

token = 'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa'
kmd_client = kmd.KMDClient(kmd_token=token, kmd_address='http://127.0.0.1:4002')
algod_client = AlgodClient(algod_token=token, algod_address='http://127.0.0.1:4001')

suggested_params = algod_client.suggested_params()
address = 'XDOV35UGGYEQHHN2UJSW4NQKS3IAFK2PYTGLK2CSUTDTXZ4EVRXTMEONEQ'
txn = PaymentTxn(sender=address, receiver=address, amt=0, sp=suggested_params)

def unencrypted_default_wallet_id() -> str:
wallets = cast(list[dict[str, str]], kmd_client.list_wallets())
for wallet in wallets:
if wallet['name'] == 'unencrypted-default-wallet':
return wallet['id']
raise Exception('not found')

wallet_handle = kmd_client.init_wallet_handle(id=unencrypted_default_wallet_id(), password='')
# works when `signing_address` is None
signed_txn = kmd_client.sign_transaction(handle=wallet_handle, password='', txn=txn)
print('*** SUCCESSFULLY SIGNED TRANSACTION ***')
pprint(signed_txn.dictify())

print('*** Signing transaction with a `signing_address` arg fails')
# fails when `signing_address` is specified
signed_txn = kmd_client.sign_transaction(handle=wallet_handle, password='', txn=txn, signing_address=address) # raises exception
```

### Your environment
py-algorand-sdk==2.0.0

./sandbox version

algod version
12885032963
3.2.3.stable [rel/stable] (commit #d2289a52)
go-algorand is licensed with AGPLv3.0
source code available at https://github.com/algorand/go-algorand

Indexer version
2.8.0-dev.unknown compiled at 2022-01-21T13:24:39+0000 from git hash cf2a3873600a95c5740f0a854ecd3e5636bb4afb (modified)

Postgres version
postgres (PostgreSQL) 13.4