Fix Block to allow regtest target (easiest possible target)

src/block.js

@@ -145,7 +145,7 @@ Block.calculateTarget = function (bits) {
   const exponent = ((bits & 0xff000000) >> 24) - 3
   const mantissa = bits & 0x007fffff
   const target = Buffer.alloc(32, 0)
-  target.writeUInt32BE(mantissa, 28 - exponent)
+  target.writeUIntBE(mantissa, 29 - exponent, 3)
   return target
 }
 

test/fixtures/block.json

@@ -19,6 +19,10 @@
     {
       "bits": "cffca00",
       "expected": "00000000000000000000000000000000000000007fca00000000000000000000"
+    },
+    {
+      "bits": "207fffff",
+      "expected": "7fffff0000000000000000000000000000000000000000000000000000000000"
     }
   ],
   "valid": [