Allow HTML files with meta tags that don't have a "name" and "content" f...

...ield to be read by the HTMLReader.

For example, `<meta charset="utf-8" />` might appear in an HTML document.  Without this change, this causes an exception.

I have followed all steps in "How to contribute" except Python 3.2 tests.  Test output is identical to the current master 675d6c81cd6ca88c8018bd608c46735ab29ba8b5 (i.e. not perfect!)

I found this problem while developing an experimental site that had HTML5 files passing through Pelican as well as reST.