add test for local fs download

common/step_download.go

@@ -179,8 +179,9 @@ func (s *StepDownload) download(ctx context.Context, ui packer.Ui, source string
 		// subdirectory command, which it most likely isn't.
 		src = filepath.Clean(u.String())
 		if _, err := os.Stat(filepath.Clean(u.Path)); err != nil {
-			// Cleaned path isn't actually present on system so it must be some
+			// Cleaned path isn't present on system so it must be some other
-			// other weird thing; see if go-getter can figure it out.
+			// scheme. Don't error right away; see if go-getter can figure it
+			// out.
 			src = u.String()
 		}
 	}

common/step_download_test.go

@@ -71,6 +71,14 @@ func TestStepDownload_Run(t *testing.T) {
 				toSha1(abs(t, "./test-fixtures/root/another.txt")) + ".lock",
 			},
 		},
+		{"double slashes on a local filesystem passes",
+			fields{Url: []string{abs(t, "./test-fixtures/root//another.txt")}},
+			multistep.ActionContinue,
+			[]string{
+				toSha1(abs(t, "./test-fixtures/root//another.txt")),
+				toSha1(abs(t, "./test-fixtures/root//another.txt")) + ".lock",
+			},
+		},
 		{"none checksum works, without a checksum",
 			fields{Url: []string{abs(t, "./test-fixtures/root/another.txt")}, ChecksumType: "none"},
 			multistep.ActionContinue,