APIcast authorising requests when Basic auth credentials are not exact

Issue

Given some valid Basic Authorization credentials: YXBwX2tleTpwYXNzd29yZA== APIcast authorizes requests even if additional characters (that make the base64 format invalid) are appended to the credentials.
The following request would be successful:
```
curl -k "https://apicast-staging.testing:443/" -H "Authorization: Basic YXBwX2tleTpwYXNzd29yZA==foo"
```

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