Correctly handle the case when PayPal returns token expires_in field as a string

Credit goes to oauth2 package.
https://github.com/golang/oauth2/blob/master/internal/token.go#L62

types.go

@@ -1,6 +1,7 @@
 package paypalsdk
 
 import (
+	"encoding/json"
 	"fmt"
 	"io"
 	"net/http"
@@ -454,12 +455,14 @@ type (
 		Phone         string `json:"phone,omitempty"`
 	}
 
+	expirationTime int64
+
 	// TokenResponse is for API response for the /oauth2/token endpoint
 	TokenResponse struct {
 		RefreshToken string         `json:"refresh_token"`
 		Token        string         `json:"access_token"`
 		Type         string         `json:"token_type"`
-		ExpiresIn    int64  `json:"expires_in"`
+		ExpiresIn    expirationTime `json:"expires_in"`
 	}
 
 	// Transaction struct
@@ -544,3 +547,17 @@ func (t JSONTime) MarshalJSON() ([]byte, error) {
 	stamp := fmt.Sprintf(`"%s"`, time.Time(t).UTC().Format(time.RFC3339))
 	return []byte(stamp), nil
 }
+
+func (e *expirationTime) UnmarshalJSON(b []byte) error {
+	var n json.Number
+	err := json.Unmarshal(b, &n)
+	if err != nil {
+		return err
+	}
+	i, err := n.Int64()
+	if err != nil {
+		return err
+	}
+	*e = expirationTime(i)
+	return nil
+}