Adding the header option to CurlDownloadStrategy.

It's easy to access github private resources later through specific `header` settings.
e.g.
```
cask 'XXXX' do
...
  url "https://raw.githubusercontent.com/....",
      header : "Authorization: token #{ENV['HOMEBREW_GITHUB_API_TOKEN']}"
...
end
 ```

Library/Homebrew/download_strategy.rb

@@ -421,6 +421,8 @@ class CurlDownloadStrategy < AbstractFileDownloadStrategy
 
     args += ["--user", meta.fetch(:user)] if meta.key?(:user)
 
+    args += ["-H", meta.fetch(:header)] if meta.key?(:header)
+
     args
   end