Fix math/big.Int Does not Marshal (#194)

* fix mashalling of math/big.Int

* Revert BigIntType. It is not needed.

ion/marshal.go

@@ -384,6 +384,9 @@ func (m *Encoder) encodeStruct(v reflect.Value) error {
 	if t == decimalType {
 		return m.encodeDecimal(v)
 	}
+	if t == bigIntType {
+		return m.encodeBigInt(v)
+	}
 
 	if err := m.w.BeginStruct(); err != nil {
 		return err
@@ -445,12 +448,18 @@ func (m *Encoder) encodeTimeDate(v reflect.Value) error {
 	return m.w.WriteTimestamp(timestamp)
 }
 
-// EncodeDecimal encodes an ion.Decimal to the output writer as an Ion decimal.
+// encodeDecimal encodes an ion.Decimal to the output writer as an Ion decimal.
 func (m *Encoder) encodeDecimal(v reflect.Value) error {
 	d := v.Addr().Interface().(*Decimal)
 	return m.w.WriteDecimal(d)
 }
 
+// encodeBigInt encodes a math/big.Int to the output writer as a big.Int.
+func (m *Encoder) encodeBigInt(v reflect.Value) error {
+	b := v.Addr().Interface().(*big.Int)
+	return m.w.WriteBigInt(b)
+}
+
 func (m *Encoder) encodeWithAnnotation(v reflect.Value, fields []field) error {
 	original := v
 	for _, field := range fields {

ion/marshal_test.go

@@ -18,6 +18,7 @@ package ion
 import (
 	"bytes"
 	"math"
+	"math/big"
 	"strings"
 	"testing"
 	"time"
@@ -42,6 +43,10 @@ func TestMarshalText(t *testing.T) {
 	test(byte(42), "42")
 	test(-42, "-42")
 	test(uint64(math.MaxUint64), "18446744073709551615")
+	bigIntPos, _ := new(big.Int).SetString("123456789012345678901234567890", 10)
+	test(bigIntPos, "123456789012345678901234567890")
+	bigIntNeg, _ := new(big.Int).SetString("-123456789012345678901234567890", 10)
+	test(bigIntNeg, "-123456789012345678901234567890")
 	test(math.MinInt64, "-9223372036854775808")
 
 	test(42.0, "4.2e+1")