Content-Type inferred from body is overridden if headers are given as object

##  Actual Behaviour
When `fetch` as below,

 ```
fetch("/foo", {
  method: "POST",
  headers: {
    "Accept": "text/html"
  },
  body: new URLSearchParams("bar=1")
})
```

the request's `Content-Type` header value is `text/plain;charset=UTF-8` since #798.

## Expected behavior
The request's `Content-Type` header value is `application/x-www-form-urlencoded;charset=UTF-8`.

