Don't set HTTP_IF_NONE_MATCH to nil if etags missing

Similar to the problem I fixed in a previous PR with HTTP_IF_MODIFIED_SINCE being set to nil.  I assume this is something that changed in a newer version of Rails, that it no longer accepts nil for these header values.

rake test
.................................................................................................................................................................................................................................................................................................................................................................
Finished in 1.295495 seconds.

353 tests, 1095 assertions, 0 failures, 0 errors