Proper handling of non breaking space

The parser is incorrect when handling text with a trailing &nbsp; inside emphasis such as <em>. Browsers usually convert a trailing space into a HTML entity. However, it seems that nokogiri and reverse_markdown together converts it back to a space, which mess up the markdown.

Works well:
`ReverseMarkdown.convert '<em>feelings: </em>'`
`=> " *feelings:* "`

Not valid markdown:
`ReverseMarkdown.convert '<em>feelings:&nbsp;</em>'`
`=> " *feelings: * "`

I've added a cleanup method to handle this scenario and convert the UTF-8 non-breaking space into a &nbsp;. I think that is better than trimming it, as the &nbsp; seems to be intentional space to be shown.