Allow converting intervals with months and days to duration

It's a lossy conversion but so is numeric to float.

fixes #42

interval.go

@@ -16,6 +16,8 @@ const (
 	microsecondsPerSecond = 1000000
 	microsecondsPerMinute = 60 * microsecondsPerSecond
 	microsecondsPerHour   = 60 * microsecondsPerMinute
+	microsecondsPerDay    = 24 * microsecondsPerHour
+	microsecondsPerMonth  = 30 * microsecondsPerDay
 )
 
 type Interval struct {
@@ -67,10 +69,8 @@ func (src *Interval) AssignTo(dst interface{}) error {
 	case Present:
 		switch v := dst.(type) {
 		case *time.Duration:
-			if src.Days > 0 || src.Months > 0 {
-				return errors.Errorf("interval with months or days cannot be decoded into %T", dst)
-			}
-			*v = time.Duration(src.Microseconds) * time.Microsecond
+			us := int64(src.Months)*microsecondsPerMonth + int64(src.Days)*microsecondsPerDay + src.Microseconds
+			*v = time.Duration(us) * time.Microsecond
 			return nil
 		default:
 			if nextDst, retry := GetAssignToDstType(dst); retry {

interval_test.go

@@ -2,9 +2,12 @@ package pgtype_test
 
 import (
 	"testing"
+	"time"
 
 	"github.com/jackc/pgtype"
 	"github.com/jackc/pgtype/testutil"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func TestIntervalTranscode(t *testing.T) {
@@ -61,3 +64,11 @@ func TestIntervalNormalize(t *testing.T) {
 		},
 	})
 }
+
+func TestIntervalLossyConversionToDuration(t *testing.T) {
+	interval := &pgtype.Interval{Months: 1, Days: 1, Status: pgtype.Present}
+	var d time.Duration
+	err := interval.AssignTo(&d)
+	require.NoError(t, err)
+	assert.EqualValues(t, int64(2678400000000000), d.Nanoseconds())
+}