[Java.Interop] JavaException.InnerException should elide Proxies.

Fixes: #1

JavaProxyThrowable is an internal type, and as such it shouldn't
"leak" if at all possible, e.g. if someone creates a
java.lang.Throwable in which the `cause` is a managed exception type
such as InvalidOperationException.

Previously, Exception.InnerException would have been the
JavaProxyThrowable, as JavaException._GetCause() would lookup a
JavaException instance. Returning a JavaProxyThrowable is ~useless;
it's not a public API, and thus nothing useful can be done with it
(short of resorting to Reflection).

Fix JavaException._GetCause() so that it instead uses
JavaVM.GetExceptionForThrowable(), which automatically "unwraps"
JavaProxyThrowable instances to return the wrapped Exception.

src/Java.Interop/Java.Interop/JavaException.cs

@@ -142,11 +142,11 @@ static string _GetMessage (JniReferenceSafeHandle handle)
 			return JniEnvironment.Strings.ToString (s, JniHandleOwnership.Transfer);
 		}
 
-		static JavaException _GetCause (JniReferenceSafeHandle handle)
+		static Exception _GetCause (JniReferenceSafeHandle handle)
 		{
 			var m = _members.InstanceMethods.GetMethodID ("getCause\u0000()Ljava/lang/Throwable;");
 			var e = m.CallVirtualObjectMethod (handle);
-			return JniEnvironment.Current.JavaVM.GetObject<JavaException> (e, JniHandleOwnership.Transfer);
+			return JniEnvironment.Current.JavaVM.GetExceptionForThrowable (e, JniHandleOwnership.Transfer);
 		}
 
 		string _GetJavaStack (JniReferenceSafeHandle handle)

src/Java.Interop/Tests/Java.Interop/JavaExceptionTests.cs

@@ -50,12 +50,43 @@ static JniLocalReference CreateThrowable (JniType type, string message)
 
 		static void SetThrowableCause (JniType type, JniLocalReference outer, string message)
 		{
-			var i = type.GetInstanceMethod ("initCause", "(Ljava/lang/Throwable;)Ljava/lang/Throwable;");
 			using (var cause = CreateThrowable (type, message)) {
-				i.CallVirtualObjectMethod (outer, new JValue (cause))
-					.Dispose ();
+				SetThrowableCause (type, outer, cause);
+			}
+		}
+
+		static void SetThrowableCause (JniType type, JniLocalReference outer, JniReferenceSafeHandle inner)
+		{
+			var i = type.GetInstanceMethod ("initCause", "(Ljava/lang/Throwable;)Ljava/lang/Throwable;");
+			i.CallVirtualObjectMethod (outer, new JValue (inner))
+				.Dispose ();
+		}
+
+		[Test]
+		public void InnerExceptionIsNotAProxy ()
+		{
+			using (var t = new JniType ("java/lang/Throwable")) {
+				var outer = CreateThrowable (t, "Outer Exception");
+				var ex    = new InvalidOperationException ("Managed Exception!");
+				var exp   = CreateJavaProxyThrowable (ex);
+				SetThrowableCause (t, outer, exp.SafeHandle);
+				using (var e = new JavaException (outer, JniHandleOwnership.Transfer)) {
+					Assert.IsNotNull (e.InnerException);
+					Assert.AreSame (ex, e.InnerException);
+				}
+				exp.Dispose ();
 			}
 		}
+
+		static JavaException CreateJavaProxyThrowable (Exception value)
+		{
+			var JavaProxyThrowable_type = typeof(JavaObject)
+				.Assembly
+				.GetType ("Java.Interop.JavaProxyThrowable", throwOnError :true);
+			var proxy   = (JavaException) Activator.CreateInstance (JavaProxyThrowable_type, value);
+			proxy.RegisterWithVM ();
+			return proxy;
+		}
 	}
 }