GitProxyOptions: pass correct version number

libgit2 began enforcing the correct version number in the proxy
options structure; pass the correct version.

Author: ethomson

LibGit2Sharp/Core/GitProxyOptions.cs

@@ -11,9 +11,9 @@ internal enum GitProxyType
     }
 
     [StructLayout(LayoutKind.Sequential)]
-    internal struct GitProxyOptions
+    internal class GitProxyOptions
     {
-        public uint Version;
+        public uint Version = 1;
         public GitProxyType Type;
         public IntPtr Url;
         public IntPtr CredentialsCb;