When force=yes, get_url should always download the specified file
When force=yes, get_url should always download the specified file

This is accomplished by not setting the If-Modified-Since header,
and setting "cache-control: no-cache" instead. Note that if the
file content has not changed, the module will still report that
changed=false, as the md5's of the tmp file and existing file are
compared before swapping

Fixes #5104

thaim/ansible,thaim/ansible