Return error on ping if using empty token (#11)

* return error if using empty token

monzo.go

@@ -3,6 +3,7 @@ package monzo
 import (
 	"bytes"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"math/rand"
@@ -36,6 +37,10 @@ func NewClient(token string) *Client {
 // Ping attempts to connect to the Monzo API using the given
 // client.
 func (c *Client) Ping() error {
+	if c.Token == "" {
+		return errors.New("error pinging Monzo API. Client token cannot be empty")
+	}
+
 	req, err := c.NewRequest(http.MethodGet, "ping/whoami", nil)
 	if err != nil {
 		return err

monzo_test.go

@@ -0,0 +1,22 @@
+package monzo
+
+import (
+	"fmt"
+	"testing"
+)
+
+func TestPing(t *testing.T) {
+	a1 := NewClient("")
+	err := a1.Ping()
+	if err == nil {
+		fmt.Println("expected an error when using empty string for token. Didn't get one")
+		t.FailNow()
+	}
+
+	a2 := NewClient("invalid_token")
+	err = a2.Ping()
+	if err == nil {
+		fmt.Print("expected an error when using an invalid token. Didn't get one")
+		t.FailNow()
+	}
+}