Round invoice amounts up to nearest msatoshi

So that very small amounts are rounded to 1 msatoshi
rather than to 0.

src/lib/exchange-rate.js

@@ -18,6 +18,7 @@ const toMsat = async (currency, amount) =>
   big(amount)
     .div(FIXED_RATES[currency] || await getRate(currency))
     .mul(BTC_MSAT_RATIO)
+    .round(0, 3) // round up to nearest msatoshi
     .toFixed(0)
 
 module.exports = { getRate, toMsat }

test/invoice.js

@@ -54,6 +54,13 @@ describe('Invoice API', function() {
         .expect(201)
         .expect(r => ok(r.body.msatoshi == null))
     })
+
+    it('rounds amounts up to the nearest msatoshi', () =>
+      charge.post('/invoice')
+        .send({ currency: 'BTC', amount: '0.0000000000000001' })
+        .expect(201)
+        .expect(r => eq(r.body.msatoshi, 1))
+    )
   })
 
   const mkInvoice = props =>