builder/common: Don't prepend slash to URL path if path is empty

builder/common/config.go

@@ -2,11 +2,11 @@ package common
 
 import (
 	"fmt"
-	"path/filepath"
 	"github.com/mitchellh/mapstructure"
 	"github.com/mitchellh/packer/packer"
 	"net/url"
 	"os"
+	"path/filepath"
 	"sort"
 	"strings"
 )
@@ -97,7 +97,7 @@ func DownloadableURL(original string) (string, error) {
 	// we distribute with a version of Go that fixes that bug.
 	//
 	// See: https://code.google.com/p/go/issues/detail?id=5927
-	if url.Path[0] != '/' {
+	if url.Path != "" && url.Path[0] != '/' {
 		url.Path = "/" + url.Path
 	}
 

builder/common/config_test.go

@@ -88,6 +88,16 @@ func TestDownloadableURL(t *testing.T) {
 	if u != "http://packer.io/path" {
 		t.Fatalf("bad: %s", u)
 	}
+
+	// No path
+	u, err = DownloadableURL("HTTP://packer.io")
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	if u != "http://packer.io" {
+		t.Fatalf("bad: %s", u)
+	}
 }
 
 func TestDownloadableURL_FilePaths(t *testing.T) {