remove unwanted wrap after break tag

[daxianyu]

the origin code will transfer '\n' to '<br>\n', and after every loop, it gets another '<br>'

<p>this is line1 <br>
 and this is line2</p>

<p>this is line1 <br>
<br>
 and this is line2</p>

<p>this is line1 <br>
<br>
<br>
 and this is line2</p>

[jpuri]

Thanks a lot @daxianyu

[jpuri]

Release draftjs-to-html@0.7.4 has the change.