Generate UnmarshalJSON to handle emptyStringInt types

This commit changes the code generator to generate a `UnmarshalJSON` for each
struct, so that if unmarshalled it properly handles UniFis varying integer values
via the `emptyStringInt` type.

Structs not including a field of `int` type will still have the function generated,
but it will effectively do nothing.

Fixes #18

unifi/wlan_group.generated.go

@@ -5,13 +5,15 @@ package unifi
 
 import (
 	"context"
+	"encoding/json"
 	"fmt"
 )
 
 // just to fix compile issues with the import
 var (
-	_ fmt.Formatter
 	_ context.Context
+	_ fmt.Formatter
+	_ json.Marshaler
 )
 
 type WLANGroup struct {
@@ -26,6 +28,22 @@ type WLANGroup struct {
 	Name string `json:"name,omitempty"` // .{1,128}
 }
 
+func (dst *WLANGroup) UnmarshalJSON(b []byte) error {
+	type Alias WLANGroup
+	aux := &struct {
+		*Alias
+	}{
+		Alias: (*Alias)(dst),
+	}
+
+	err := json.Unmarshal(b, &aux)
+	if err != nil {
+		return fmt.Errorf("unable to unmarshal alias: %w", err)
+	}
+
+	return nil
+}
+
 func (c *Client) listWLANGroup(ctx context.Context, site string) ([]WLANGroup, error) {
 	var respBody struct {
 		Meta meta        `json:"meta"`