Throw an error when Amount or Fee contains a decimal (Fix #31)

Thanks to @jwbusch for review

packages/ripple-binary-codec/test/tx-encode-decode-test.js

@@ -0,0 +1,124 @@
+const assert = require('assert');
+const {
+  encode,
+  decode
+} = require('../src')
+
+// Notice: no Amount or Fee
+const tx_json = {
+  Account: 'r9LqNeG6qHxjeUocjvVki2XR35weJ9mZgQ',
+  // Amount: '1000',
+  Destination: 'rHb9CJAWyB4rj91VRWn96DkukG4bwdtyTh',
+  // Fee: '10',
+
+  // JavaScript converts operands to 32-bit signed ints after doing bitwise
+  // operations. We need to convert it back to an unsigned int with >>> 0.
+  Flags: ((1 << 31) >>> 0), // tfFullyCanonicalSig
+
+  Sequence: 1,
+  TransactionType: 'Payment'
+  // TxnSignature,
+  // Signature,
+  // SigningPubKey
+};
+
+const amount_parameters_message = input => {
+  // disables the ESLint rule on the whole rest of the file
+  /* eslint-disable max-len */
+  return `${input} is an illegal amount
+
+Native values must be described in drops, a million of which equal one XRP.
+This must be an integer number, with the absolute value not exceeding 100000000000000000
+
+IOU values must have a maximum precision of 16 significant digits. They are serialized as
+a canonicalised mantissa and exponent.
+
+The valid range for a mantissa is between 1000000000000000 and 9999999999999999
+The exponent must be >= -96 and <= 80
+
+Thus the largest serializable IOU value is:
+999999999999999900000000000000000000000000000000000000000000000000000000000000000000000000000000
+
+And the smallest:
+0.000000000000000000000000000000000000000000000000000000000000000000000000000000001
+`;
+};
+
+describe('encoding and decoding tx_json', function() {
+  it('can encode tx_json without Amount or Fee', function() {
+    const encoded = encode(tx_json);
+    const decoded = decode(encoded);
+    assert.deepStrictEqual(tx_json, decoded);
+  });
+  it('can encode tx_json with Amount and Fee', function() {
+    const my_tx = Object.assign({}, tx_json, {
+      Amount: '1000',
+      Fee: '10'
+    });
+    const encoded = encode(my_tx);
+    const decoded = decode(encoded);
+    assert.deepStrictEqual(my_tx, decoded);
+  });
+  it('throws when Amount is invalid', function() {
+    const my_tx = Object.assign({}, tx_json, {
+      Amount: '1000.001',
+      Fee: '10'
+    });
+    assert.throws(() => {
+      encode(my_tx);
+    }, {
+      name: 'Error',
+      message: amount_parameters_message('1000.001')
+    });
+  });
+  it('throws when Fee is invalid', function() {
+    const my_tx = Object.assign({}, tx_json, {
+      Amount: '1000',
+      Fee: '10.123'
+    });
+    assert.throws(() => {
+      encode(my_tx);
+    }, {
+      name: 'Error',
+      message: amount_parameters_message('10.123')
+    });
+  });
+  it('throws when Amount and Fee are invalid', function() {
+    const my_tx = Object.assign({}, tx_json, {
+      Amount: '1000.789',
+      Fee: '10.123'
+    });
+    assert.throws(() => {
+      encode(my_tx);
+    }, {
+      name: 'Error',
+      message: amount_parameters_message('1000.789')
+    });
+  });
+  it('throws when Amount is a number instead of a string-encoded integer',
+    function() {
+      const my_tx = Object.assign({}, tx_json, {
+        Amount: 1000.789
+      });
+      assert.throws(() => {
+        encode(my_tx);
+      },
+      {
+        name: 'Error',
+        message: 'unsupported value: 1000.789'
+      });
+    });
+  it('throws when Fee is a number instead of a string-encoded integer',
+    function() {
+      const my_tx = Object.assign({}, tx_json, {
+        Amount: 1234.56
+      });
+      assert.throws(() => {
+        encode(my_tx);
+      },
+      {
+        name: 'Error',
+        message: 'unsupported value: 1234.56'
+      });
+    });
+});