Fix incorrect reporting of block difficulty.

The nBits value for mainnet genesis was being used instead of testnet.

lib/block/blockheader.js

@@ -9,7 +9,8 @@ var Hash = require('../crypto/hash');
 var JSUtil = require('../util/js');
 var $ = require('../util/preconditions');
 
-var GENESIS_BITS = 0x1f07ffff;
+// Mainnet 0x1f07ffff, Testnet 0x2007ffff
+var GENESIS_BITS = 0x2007ffff;
 
 /**
  * Instantiate a BlockHeader from a Buffer, JSON object, or Object with