Log Message:
= 4.6.0 (20170507) =

* Added the `Tag.get_attribute_list` method, which acts like `Tag.get` for
  getting the value of an attribute, but which always returns a list,
  whether or not the attribute is a multi-value attribute.

* It's now possible to use a tag's namespace prefix when searching,
  e.g. soup.find('namespace:tag')

* Improved the handling of empty-element tags like <br> when using the
  html.parser parser.

* HTML parsers treat all HTML4 and HTML5 empty element tags (aka void
  element tags) correctly.

* Namespace prefix is preserved when an XML tag is copied. Thanks
  to Vikas for a patch and test.