Expose the Account API (RADIUS users)

* Adds Account CRUD API
* Enahnces `emptyStringInt` with a predicate for dynamically setting
  which value should be interpreted as a blank JSON payload

unifi/account_test.go

@@ -0,0 +1,37 @@
+package unifi_test
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/paultyng/go-unifi/unifi"
+	"github.com/tj/assert"
+)
+
+func TestAccountMarshalJSON(t *testing.T) {
+	for n, c := range map[string]struct {
+		expectedJSON string
+		acc          unifi.Account
+	}{
+		"empty strings": {
+			`{"vlan":"","tunnel_type":"","tunnel_medium_type":""}`,
+			unifi.Account{},
+		},
+		"response": {
+			`{"vlan":10,"tunnel_type":1,"tunnel_medium_type":1}`,
+			unifi.Account{
+				VLAN:             10,
+				TunnelType:       1,
+				TunnelMediumType: 1,
+			},
+		},
+	} {
+		t.Run(n, func(t *testing.T) {
+			actual, err := json.Marshal(&c.acc)
+			if err != nil {
+				t.Fatal(err)
+			}
+			assert.JSONEq(t, c.expectedJSON, string(actual))
+		})
+	}
+}