Fix up the seal fields in RPC output. (#1096)

Make Ethash::extra_info() work.
Seal fields now decode the RLP.

ethcore/src/ethereum/ethash.rs

@@ -93,7 +93,9 @@ impl Engine for Ethash {
 	}
 
 	/// Additional engine-specific information for the user/developer concerning `header`.
-	fn extra_info(&self, _header: &Header) -> HashMap<String, String> { HashMap::new() }
+	fn extra_info(&self, header: &Header) -> HashMap<String, String> {
+		hash_map!["nonce".to_owned() => format!("0x{}", header.nonce().hex()), "mixHash".to_owned() => format!("0x{}", header.mix_hash().hex())]
+	}
 
 	fn vm_factory(&self) -> &Factory {
 		&self.factory