Custom headers missing from redirect response

**Please describe which feature you have a question about?**
Is it possible to have headers in a redirect response? As an example, if I want to be able to set `Strict-Transport-Security: max-age=63072000; includeSubDomains; preload` to be [HSTS compliant](https://infosec.mozilla.org/guidelines/web_security#http-strict-transport-security) is that possible? I need to be able to serve that on example.com in addition to just on www.example.com. Any guidance would be much appreciated.

**Provide additional details**
In my build settings I have the below:
```yaml
customHeaders:
- pattern: '**/*'
headers:
- key: 'Strict-Transport-Security'
value: 'max-age=31536000; includeSubDomains; preload'
- key: 'X-Frame-Options'
value: 'SAMEORIGIN'
- key: 'X-XSS-Protection'
value: '1; mode=block'
- key: 'X-Content-Type-Options'
value: 'nosniff'
- key: 'Content-Security-Policy'
value: "default-src 'none'; script-src 'self';... snipped"
- key: 'Referrer-Policy'
value: 'same-origin'
```

I have the below in my redirects (domain changed):
```json
[
{
"source": "http://example.com",
"target": "https://www.example.com",
"status": "301",
"condition": null
},
{
"source": "http://www.example.com",
"target": "https://www.example.com",
"status": "301",
"condition": null
},
{
"source": "https://example.com",
"target": "https://www.example.com",
"status": "301",
"condition": null
},
{
"source": "/<*>",
"target": "/index.html",
"status": "404",
"condition": null
}
]
```

Right now when looking at the headers for http://example.com I see the below:
```http
HTTP/1.1 301 Moved Permanently
Connection: keep-alive
Content-Length: 183
Content-Type: text/html
Date: Sun, 26 Apr 2020 16:55:37 GMT
Location: https://exmple.com/
Server: CloudFront
Via: 1.1 redacted.cloudfront.net (CloudFront)
X-Amz-Cf-Id: 3yRuneR4WhJxPryPpeOnDukKPiEDsvyuz3ZuwPLYJBKmvnVlo9qW_g==
X-Amz-Cf-Pop: EWR52-C4
X-Cache: Redirect from cloudfront
```

Likewise for https://example.com:
```http
HTTP/1.1 301 Moved Permanently
Connection: keep-alive
Content-Length: 0
Date: Sun, 26 Apr 2020 17:10:22 GMT
Location: https://www.example.com/
Server: CloudFront
Via: 1.1 redacted.cloudfront.net (CloudFront)
X-Amz-Cf-Id: QrwGwtEutf550fxyPWiivTyr51MnOxd1ekIMlIZlIZ88ejX-URcI2g==
X-Amz-Cf-Pop: EWR52-C1
X-Cache: Miss from cloudfront
```

**App id:** d2gjczoto5f568

No repository files or tests are mentioned. Start by reproducing the configured customHeaders and redirects in Amplify Hosting, then compare headers on redirected and non-redirected responses; done means establishing whether configured headers should be present on redirects and documenting or correcting the behavior.

Written by the indexing model from the issue text.