Inconsistent page.date handling

[thoemmi]

Currently the handling of page.date is inconsistent:

1. If there's no date specified in a page's front-matter but is extracted from the filename, page.Bag["date"] is a DateTime
2. If there's a date specified in a page's front-matter, page.Bag["date"] is a string (as parsed by YamlDotNet)

When you're using {{ page.date | date_to_xmlschema }} on your template, and page.Bag["date"] is a string, it will be rendered as Liquid error: Object of type 'System.String' cannot be converted to type 'System.DateTime'.

This PR fixes this by always adding the date back to page.Bag["date"] as DateTime (in fact I only had to delete some code in SiteContextGenerator.cs. 😉) Two other tests relied on strings, which I fixed.

[laedit]

Excellent, thanks a lot!