HTMLToMarkdownConverter appends an extra space to strong/b elements with an existing trailing space

There is some logic to handle trailing spaces in `<b> / <strong>` elements in HTML, but there was a small bug in how the text was manipulated.  

The trailing space was stripped from `boldtext`, but was then reappended to `htmlNode.InnerText` which also includes the original trailing space.  

I've fixed the code and added a test.