fix UIWebView.delegate not set nil when the delegate is dealloc

[Ekulelu]

In our app, we got some crash like this

Application received signal 11

1 libobjc.A.dylib 0x1800bc150 _objc_msgSend + 16 2 UIKit 0x18799156c -[UIWebView webView:resource:didFinishLoadingFromDataSource:] + 84 3 CoreFoundation 0x18164ce80 invoking + 144 4 CoreFoundation 0x1815422c4 -[NSInvocation invoke] + 292 5 CoreFoundation 0x181546e9c -[NSInvocation invokeWithTarget:] + 60 6 WebKitLegacy 0x1873d4820

Application received signal 11

WebThread

1 libobjc.A.dylib 0x1800bc150 _objc_msgSend + 16 2 UIKit 0x187c33354 -[UIWebView webThreadWebView:resource:willSendRequest:redirectResponse:fromDataSource:] + 92 3 WebKitLegacy 0x187477e60 4 WebKitLegacy 0x1873d315c 5 WebCore 0x1861cdee0 WebCore::ResourceLoadNotifier::dispatchWillSendRequest(WebCore::DocumentLoader*, unsigned long, WebCore::ResourceRequest&, WebCore::ResourceResponse const&) + 232 6 WebCore 0x186f1c494 WebCore::ResourceLoader::willSendRequestInternal(WebCore::ResourceRequest&, WebCore::ResourceResponse const&) + 556 7 WebCore 0x18703cbb0 WebCore::SubresourceLoader::willSendRequestInternal(WebCore::ResourceRequest&, WebCore::ResourceResponse const&) + 252 8 WebCore 0x1861cd040 WebCore::ResourceLoader::init(WebCore::ResourceRequest const&) + 288 9 WebCore 0x1861ccdec WebCore::SubresourceLoader::startLoading() + 36 10 WebKitLegacy 0x1874b23b0

Some blog found out this is because the UIWebView.delegate's property is assign, instead of weak. So, we should set this to nil manually, which is also pointed out in Apple's document: "Important Before releasing an instance of UIWebView for which you have set a delegate, you must first set its delegate property to nil. This can be done, for example, in your dealloc method."