Google Search Console: Sitemap could not be read

## Description

Google search console cannot read Gridsome sitemap.

### Steps to reproduce

```
{
use: '@gridsome/plugin-sitemap',
options: {
cacheTime: 600000, // default
},
},
```
### Expected result

Sitemap should be accepted by Google.

### Actual result

sitemap.xml exists and can be read but Google status is: "Sitemap could not be read"

### Environment

- gridsome version: 0.6.3

- @gridsome/cli version: 0.1.0

 

- Platform:  Netlify

Start with the @gridsome/plugin-sitemap configuration shown in the issue and reproduce the generated sitemap on Netlify using Gridsome 0.6.3. Compare the sitemap behavior with Google Search Console's acceptance requirements; done means Google can read and accept the generated sitemap.

Written by the indexing model from the issue text.