common: downloads can be HTTPS [GH-587]

CHANGELOG.md

@@ -7,6 +7,7 @@ FEATURES:
 
 IMPROVEMENTS:
 
+* builder/vmware,builder/virtualbox: ISO urls can now be https [GH-587]
 * builder/virtualbox: Can now disable guest addition download entirely
   by setting "guest_additions_mode" to "disable" [GH-580]
 

common/download.go

@@ -74,6 +74,7 @@ func NewDownloadClient(c *DownloadConfig) *DownloadClient {
 	if c.DownloaderMap == nil {
 		c.DownloaderMap = map[string]Downloader{
 			"http":  new(HTTPDownloader),
+			"https": new(HTTPDownloader),
 		}
 	}