Fix Block model to match server: nest header with camelCase keys

The block endpoint returns block id / prev / timestamp nested under a
"header" object with camelCase keys (blockId, dcId, prevId, prevProof,
timestamp) and a proof of just {proof}. The previous flat snake_case
Block fields never matched the response and always deserialized empty.
Add a BlockHeader struct, nest it in Block, and make Proof.Scheme
omitempty. Verified live against a dev chain.

models/models.go

@@ -122,17 +122,22 @@ type SmartContractExecutionInfo struct {
 
 type Block struct {
 	Version      string      `json:"version"`
-	ID           string     `json:"block_id"`
-	Timestamp    string     `json:"timestamp"`
-	PrevID       string     `json:"prev_id"`
-	PrevProof    string     `json:"prev_proof"`
+	Header       BlockHeader `json:"header"`
 	Transactions []string    `json:"transactions"`
 	Proof        BlockProof  `json:"proof"`
 }
 
+type BlockHeader struct {
+	BlockId   string `json:"blockId"`
+	DcId      string `json:"dcId"`
+	PrevId    string `json:"prevId"`
+	PrevProof string `json:"prevProof"`
+	Timestamp string `json:"timestamp"`
+}
+
 type BlockProof struct {
-	Scheme string `json:"scheme"`
 	Proof  string `json:"proof"`
+	Scheme string `json:"scheme,omitempty"`
 	Nonce  int64  `json:"nonce,omitempty"`
 }