Fix XMLHttpRequest to set the responseType correctly

"xhr.responseType" was mis-spelled as "responsetype" (lowercase t)

Contributed by tbroyer
Fixes issue 7386
Review at https://gwt-code-reviews.appspot.com/1820806/

Review by: cromwellian@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@11264 8db76d5a-ed1c-0410-87a9-c151d255dfc7