Correct parsing Upload-Concat for final uplaods

Addresses https://github.com/tus/tus-resumable-upload-protocol/issues/123

concat_test.go

@@ -118,6 +118,9 @@ func TestConcat(t *testing.T) {
 				Method: "POST",
 				ReqHeader: map[string]string{
 					"Tus-Resumable": "1.0.0",
+					// A space between `final;` and the first URL should be allowed due to
+					// compatibility reasons, even if the specification does not define
+					// it. Therefore this character is included in this test case.
 					"Upload-Concat": "final; http://tus.io/files/a /files/b/",
 				},
 				Code: http.StatusCreated,

unrouted_handler.go

@@ -341,8 +341,11 @@ func (handler *UnroutedHandler) HeadFile(w http.ResponseWriter, r *http.Request)
 	if info.IsFinal {
 		v := "final;"
 		for _, uploadID := range info.PartialUploads {
-			v += " " + handler.absFileURL(r, uploadID)
+			v += handler.absFileURL(r, uploadID) + " "
 		}
+		// Remove trailing space
+		v = v[:len(v)-1]
+
 		w.Header().Set("Upload-Concat", v)
 	}