Error when adding cutom feeRate ---> in pwCore.send(address, amount, {feeRate: 1100})

[dxnght]

• I'm submitting a ...

• bug report
• feature request

• Summary
  The problem is in pwCore.send() method when adding {feeRate: 1100} (for example)
  Related to Builder.calcFee() --->
  const fee = (feeRate / FEE_BASE) * txSize;
  return new models_1.Amount(fee.toString(), models_1.AmountUnit.shannon);

Returned error: SyntaxError: Cannot convert 742.5000000000001 to a BigInt

• Other information (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. StackOverflow, personal fork, etc.)

I think the problem is with fee.toString()
I'm using ${value} when creating a new Amount()
Need to be reproduced and re-checked

@sking789 @louzhixian