[SSWebView] Fixed AJAX-based page loading bug & webViewDidStartLoadingPage behavior

(1) AJAX-based page loading fix: d0db84e

I found the issue in Youtube website which has changed page loading system recently.
It seems injecting testDOM causes the page unable to load,
so I avoided it by adding `shouldTestDOMLoaded` property and set to `NO`.
(I kept default = `YES` as it was before)


(2) webViewDidStartLoadingPage fix: 4b9cee3

When loading same URL but only hash is different,
webView will not call `-webViewDidStartLoad:` and `-webViewDidFinishLoad:`,
so should `-webViewDidStartLoadingPage:`.