Let Nil be converted to int, but other values cannot.

src/main/java/lua/value/LNil.java

@@ -15,4 +15,9 @@ public final class LNil extends LValue {
 	public LString luaGetType() {
 		return TYPE_NAME;
 	}
+
+	public int luaAsInt() {
+		return 0;
+	}
+
 }

src/main/java/lua/value/LValue.java

@@ -105,6 +105,7 @@ public class LValue {
 
 	/** Return value as an integer */
 	public int luaAsInt() {
+		luaUnsupportedOperation();
 		return 0;
 	}