定义 UnityWebRequest 要通信的目标 URL。
This property cannot be set after calling SendWebRequest.
如果 UnityWebRequest 遇到并遵循重定向，此属性将更新为 UnityWebRequest 被重定向到的 URL。
输入 URL 时，首选绝对 URL。但如果您输入部分 URL，系统将遵循以下规则：
If the input URL starts with two slashes (//), then the input is assumed to be a domain and path intended for use over HTTPS.
On non-WebGL platforms, the system will prepend https:. On WebGL, the system will inherit the scheme of the path by which the Unity WebGL application is being accessed.
示例：如果通过 https, 来访问 WebGL 应用程序，系统将前置 https:.。如果通过 http, 来访问 WebGL 应用程序，系统将前置 http:.。
If the input URL starts with a single slash (/), then the system assumes the inout is a path relative to the current domain on which the Unity application is running. On non-WebGL platforms, the system will prepend https://localhost to the URL.
On WebGL, the system will prepend the scheme and host of the path by which the Unity WebGL application is being accessed. For example, if the Unity WebGL app is being accessed via https://unity3d.com/myapp, then the system will prepend https://unity3d.com to relative paths.
If neither of the above rules apply, the system validates your input string via the built-in System.Uri class. If this class throws a URIFormatException, the system attempts to append the input string to the absolute URL by which the Unity app is being accessed. (see above)
如有其他异常，将被重新抛出。