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. 

`"<b>hello </b>"` should be converted to `"**hello** "` but retains the first space as well so it is `"**hello ** "` 

The trailing space was stripped from `boldtext`, but was then reappended to the original value `htmlNode.InnerText` (which also includes the original trailing space).  

I've fixed the code and added a test.