Restore force in copy since arguments are no longer clobbered

Fixes #2388.

library/copy

@@ -105,8 +105,8 @@ def main():
     md5sum_dest = None
 
     if os.path.exists(dest):
-        # if not force:
-        #    module.exit_json(msg="file already exists and force is set (%s)" % force, src=src, dest=dest, changed=False)
+        if not force:
+            module.exit_json(msg="file already exists", src=src, dest=dest, changed=False)
         if (os.path.isdir(dest)):
             basename = os.path.basename(src)
             dest = os.path.join(dest, basename)