Fetch: fixed handling of Content-Length header when body is provided.

Fetch: fixed handling of Content-Length header when body is provided.

body value length takes precedence over Content-Length from header list.

    https://fetch.spec.whatwg.org/#http-network-or-cache-fetch

    Let contentLength be httpRequest’s body’s length, if httpRequest’s body
is non-null; otherwise null.

    Let contentLengthHeaderValue be null.

    If httpRequest’s body is null and httpRequest’s method is `POST` or
`PUT`, then set contentLengthHeaderValue to `0`.

    If contentLength is non-null, then set contentLengthHeaderValue to
contentLength, serialized and isomorphic encoded.

    If contentLengthHeaderValue is non-null, then append (`Content-Length`,
contentLengthHeaderValue) to httpRequest’s header list.

This fixes #930 issue in Github.