allow fee to be set to zero

src/wallet.js

@@ -184,7 +184,7 @@ var Wallet = function (seed, options) {
           totalInValue += output.value
           if(totalInValue < value) continue;
 
-          var fee = fixedFee || estimateFeePadChangeOutput(tx)
+          var fee = fixedFee == undefined ? estimateFeePadChangeOutput(tx) : fixedFee
           if(totalInValue < value + fee) continue;
 
           var change = totalInValue - value - fee

test/wallet.js

@@ -434,6 +434,15 @@ describe('Wallet', function() {
         assert.deepEqual(tx.ins[1].outpoint, { hash: fakeTxHash(2), index: 1 })
       })
 
+      it('allows fee to be set to zero', function(){
+        value = 520000
+        var fee = 0
+        var tx = wallet.createTx(to, value, fee)
+
+        assert.equal(tx.ins.length, 1)
+        assert.deepEqual(tx.ins[0].outpoint, { hash: fakeTxHash(3), index: 0 })
+      })
+
       it('ignores spent outputs', function(){
         utxo.push(
           {