Fixed handling absent href attribute in base tag

The HTML5 spec states that the href attribute is optional for the
base tag. Fixed the DomCrawler::addHtmlContent() method to support this

See here and here:
http://www.w3.org/TR/html-markup/base.html#base.attrs.href
http://www.whatwg.org/specs/web-apps/current-work/multipage/semantics.html#the-base-element