[whatwg/fetch] TypeError new Headers(obj) (#300)

Works:
```javascript
var gh_headers = new window.Headers()
gh_headers.append('Accept', 'application/vnd.github.v3.html')
```

Doesn't Work:
```javascript
var gh_headers = new window.Headers({ 'Accept': 'application/vnd.github.v3.html' })
// Uncaught TypeError: Failed to construct 'Headers': Invalid value
```

Am I missing something?

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