UriBuilder is more forgiving (than Uri) when parsing string uri

UriBuilder is more forgiving (than Uri) when parsing string uri

* System/UriBuilder.cs: Refactor constructors to share Initialization
methods. Allow a second chance to .ctor(string) to parse an URI by
assuming it's an HTTP URI

* Test/System/UriBuilderTest.cs: Add test case with string URI that do
not provide a scheme - UriBuilder will default them to http:// even if
Uri would not parse them