rpc: fix blockheader marshalling in getblockheader RPC server

Problem: wrong json marshalling of `nextconsensus` field of
result.Header

Solution: change field type from util.uint160 to address string

pkg/rpc/response/result/block_header.go

@@ -6,6 +6,7 @@ import (
 	"github.com/nspcc-dev/neo-go/pkg/core"
 	"github.com/nspcc-dev/neo-go/pkg/core/block"
 	"github.com/nspcc-dev/neo-go/pkg/core/transaction"
+	"github.com/nspcc-dev/neo-go/pkg/encoding/address"
 	"github.com/nspcc-dev/neo-go/pkg/io"
 	"github.com/nspcc-dev/neo-go/pkg/util"
 )
@@ -22,7 +23,7 @@ type (
 		Timestamp     uint32              `json:"time"`
 		Index         uint32              `json:"index"`
 		Nonce         string              `json:"nonce"`
-		NextConsensus util.Uint160        `json:"nextconsensus"`
+		NextConsensus string              `json:"nextconsensus"`
 		Script        transaction.Witness `json:"script"`
 		Confirmations uint32              `json:"confirmations"`
 		NextBlockHash *util.Uint256       `json:"nextblockhash,omitempty"`
@@ -40,7 +41,7 @@ func NewHeader(h *block.Header, chain core.Blockchainer) Header {
 		Timestamp:     h.Timestamp,
 		Index:         h.Index,
 		Nonce:         strconv.FormatUint(h.ConsensusData, 16),
-		NextConsensus: h.NextConsensus,
+		NextConsensus: address.Uint160ToString(h.NextConsensus),
 		Script:        h.Script,
 		Confirmations: chain.BlockHeight() - h.Index + 1,
 	}

pkg/rpc/server/server_test.go

@@ -441,7 +441,7 @@ var rpcTestCases = map[string][]rpcTestCase{
 					Timestamp:     header.Timestamp,
 					Index:         header.Index,
 					Nonce:         strconv.FormatUint(header.ConsensusData, 16),
-					NextConsensus: header.NextConsensus,
+					NextConsensus: address.Uint160ToString(header.NextConsensus),
 					Script:        header.Script,
 					Confirmations: e.chain.BlockHeight() - header.Index + 1,
 				}