allow feePerKb to be set to zero

src/transaction.js

@@ -382,7 +382,8 @@ Transaction.prototype.estimateFee = function(feePerKb){
   var uncompressedInSize = 180
   var outSize = 34
   var fixedPadding = 34
-  var feePerKb = feePerKb || Transaction.feePerKb
+
+  if(feePerKb == undefined) feePerKb = Transaction.feePerKb
   var size = this.ins.length * uncompressedInSize + this.outs.length * outSize + fixedPadding
 
   return feePerKb * Math.ceil(size / 1000)

test/transaction.js

@@ -194,6 +194,11 @@ describe('Transaction', function() {
         var tx = Transaction.deserialize(fixtureTx2Hex)
         assert.equal(tx.estimateFee(10000), 10000)
       })
+
+      it('allow feePerKb to be set to 0', function(){
+        var tx = Transaction.deserialize(fixtureTx2Hex)
+        assert.equal(tx.estimateFee(0), 0)
+      })
     })
   })