Wrap tun persist property and add assertion to ipv6 setter

OpenVPN Adapter/OpenVPNConfiguration.h

@@ -64,4 +64,9 @@ typedef NS_ENUM(NSInteger, IPv6Preference) {
  */
 @property (nonatomic) NSInteger connectionTimeout;
 
+/**
+ Keep tun interface active during pauses or reconnections
+ */
+@property (nonatomic) BOOL tunPersist;
+
 @end

OpenVPN Adapter/OpenVPNConfiguration.mm

@@ -118,6 +118,10 @@ using namespace openvpn;
         case IPv6PreferenceDefault:
             _config.ipv6 = "default";
             break;
+            
+        default:
+            NSAssert(NO, @"Incorrect IPv6Preference value");
+            break;
     }
 }
 
@@ -129,4 +133,12 @@ using namespace openvpn;
     _config.connTimeout = connectionTimeout;
 }
 
+- (BOOL)tunPersist {
+    return _config.tunPersist;
+}
+
+- (void)setTunPersist:(BOOL)tunPersist {
+    _config.tunPersist = tunPersist;
+}
+
 @end