fix(builder): the value of a variable could be a number

sorry, I meant as an actual patch to the existing ClosingBuffer:

```
diff --git a/builder/utils_test.go b/builder/utils_test.go
index 5f38b7a..e44b90f 100644
--- a/builder/utils_test.go
+++ b/builder/utils_test.go
@@ -59,7 +59,7 @@ func TestParseConfigGood(t *testing.T) {
        resp := &http.Response{
                Body: &ClosingBuffer{bytes.NewBufferString(`{"owner": "test",
                        "app": "example-go",
-                       "values": {"FOO": "bar"},
+                       "values": {"FOO": "bar", "CAR": 1234},
                        "memory": {},
                        "cpu": {},
                        "tags": {},
@@ -76,7 +76,15 @@ func TestParseConfigGood(t *testing.T) {
        }

        if config.Values["FOO"] != "bar" {
-               t.Errorf("expected FOO='bar', got FOO='%s'", config.Values["FOO"])
+               t.Errorf("expected FOO='bar', got FOO='%v'", config.Values["FOO"])
+       }
+
+       if car, ok := config.Values["CAR"].(float64); ok {
+               if car != 1234 {
+                       t.Errorf("expected CAR=1234, got CAR=%d", config.Values["CAR"])
+               }
+       } else {
+               t.Error("expected CAR to be of type float64")
        }
 }
```