IIS 8.5 doesn't accept PROPPATCH with empty self-closing prop to remove

[kalmykov-sergey]

IIS returns 400 Bad Request for

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<propertyupdate xmlns="DAV:">
   <set>
      <prop>
         <s:filename xmlns:s="SAR:">webdav.txt</s:filename>
      </prop>
   </set>
   <remove>
      <prop/>
   </remove>
</propertyupdate>

Non-existing property works, no remove tag for empty also works.
Simple patch in com/github/sardine/impl/SardineImpl.java:559 is tested for IIS 8.5

if (!removeProps.isEmpty()) {