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
```
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