Add tests and continue integration test

offer_test.go

@@ -0,0 +1,15 @@
+package ebay_test
+
+import (
+	"net/http"
+	"testing"
+
+	"github.com/jybp/ebay"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestOptBuyMarketplace(t *testing.T) {
+	r, _ := http.NewRequest("", "", nil)
+	ebay.OptBuyMarketplace("EBAY_US")(r)
+	assert.Equal(t, "EBAY_US", r.Header.Get("X-EBAY-C-MARKETPLACE-ID"))
+}