Removed trailing whitespace in link definition (kramdown output)

Avoids a trailing whitespace in link definitions without a title
when converting to kramdown format:

```
require 'kramdown'
htmldoc =<<HTML
<p><a href="http://test1.com/">Test 1</a></p>
<p><a href="http://test2.com/" title="Test 2">Test 2</a></p>
HTML
puts Kramdown::Document.new(htmldoc, :input => :html).to_kramdown
```
Output:

    [Test 1][1]

    [Test 2][2]



    [1]: http://test1.com/ 
    [2]: http://test2.com/ "Test 2"

with a whitespace at the end of link definition 1.