SSPI implementation converts SPN to Lowercase, Causing Authentication Failure with Non-Windows Machines

**Description:**
I am using a non-Windows machine with Kerberos protection, where the Service Principal Name (SPN) is case-sensitive. The SPN in my case is HTTP/myserver. However, Kerberos.NET's `SspiSecurityContext.InitializeSecurityContext` method converts the SPN to lowercase (http/myserver), causing authentication to fail because the server cannot verify the token with the altered SPN.

This behavior breaks compatibility with non-Windows Kerberos implementations, as many UNIX-based systems enforce case sensitivity for SPNs.

**Steps to Reproduce:**

Set up a Kerberos-protected non-Windows machine with an SPN HTTP/myserver.
Use Kerberos.NET SSPI to authenticate using the SPN HTTP/myserver.
Observe that the SPN is converted to http/myserver, resulting in authentication failure on the server.
Expected Behavior:
The SPN string provided to the `SspiSecurityContext.InitializeSecurityContext` method should remain in its original case to ensure compatibility with case-sensitive non-Windows servers.

**Actual Behavior:**
The SPN string is converted to lowercase (http/myserver), leading to token verification failure on the non-Windows server.

**Environment:**

Kerberos.NET version: [e.g., 4.6.70]
.NET version: [e.g., .NET 8.0]
Operating System: [e.g., Windows 10, Ubuntu 20.04]
Additional Information:

SPNs are case-insensitive on Windows systems but case-sensitive on many non-Windows systems.
Refer to [Microsoft documentation](https://learn.microsoft.com/en-us/previous-versions/windows/it-pro/windows-server-2012-r2-and-2012/cc731241(v=ws.11)) for more information:
"Service Principal Names (SPNs) are not case sensitive when used by Microsoft Windows-based computers. However, many UNIX-based systems are case-sensitive and require the proper case to function properly."

A potential fix would be to avoid altering the case of the SPN string in `SspiSecurityContext .InitializeSecurityContext`.