The current logic for curl_download tries to restart any existing
partial download, but this fails for some servers when the file is
already fully downloaded, returning a status code 416. This updates the
function to check for the file being already downloaded and if so bails
early.
The following command used to fail when run more than once but now works
as expected:
```
brew cask info https://raw.githubusercontent.com/homebrew/homebrew-cask/master/Casks/docker.rb
```
In a number of Cask specs, the value of the `homepage` stanza is currently set
to https://example.com. As of 2018-11-28, the TLS certificate served by
example.com seems to be expired, possibly due to an oversight on ICANN’s side.
While the certificate is certainly going to be renewed soon, it would be
desirable for Homebrew’s test result to be less dependent on ICANN’s actions.
This commit changes the homepages of all test Casks to http://brew.sh, whose
domain and TLS certificate are both controlled by Homebrew.
