Remove potentially redundant HEAD request

[eritiro]

When you check if a file exists using Fog, you can potentially end up with an extra HEAD request to the remote server: one request to check its existence and another to read the file.

Consider the following code:

if fog_file.exists?
  fog_file.read
end

It will produce two HEAD request and one GET
With this PR, it will produce only one HEAD and one GET.

[mshibuya]

Agreed, thanks.