`get-url` should just `GET` files

[skshetry]

If an endpoint does not support HEAD, but supports GET, dvc get-url will fail to work.

Eg:

$ dvc get-url https://api.travis-ci.com/v3/job/363241860/log.txt -v
2020-07-21 17:23:50,482 ERROR: failed to get 'https://api.travis-ci.com/v3/job/363241860/log.txt' - dependency 'https://api.travis-ci.com/v3/job/363241860/log.txt' does not exist
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Traceback (most recent call last):
  File "/home/saugat/repos/iterative/dvc/dvc/command/get_url.py", line 17, in run
    Repo.get_url(self.args.url, out=self.args.out)
  File "/home/saugat/repos/iterative/dvc/dvc/repo/get_url.py", line 19, in get_url
    dep.save()
  File "/home/saugat/repos/iterative/dvc/dvc/output/base.py", line 253, in save
    raise self.DoesNotExistError(self)
dvc.dependency.base.DependencyDoesNotExistError: dependency 'https://api.travis-ci.com/v3/job/363241860/log.txt' does not exist
------------------------------------------------------------

Just commenting dep.save() makes it work, as it does a HEAD request first which fails.