Prefer Content-Length over Transfer-Encoding: chunked for content=<file-like> cases.

[tomchristie]

Closes #1490

Step 1: Add failing test case for content=io.BytesIO(...):

def test_bytesio_content():
    headers, stream = encode_request(content=io.BytesIO(b"Hello, world!"))
    assert isinstance(stream, typing.Iterable)
    assert not isinstance(stream, typing.AsyncIterable)

    content = b"".join([part for part in stream])

    assert headers == {"Content-Length": "13"}  # Currently returns {'Transfer-Encoding': 'chunked'}
    assert content == b"Hello, world!"

Step 2: Refactor peek_filelike_length to return Optional[int], rather than raising an exception when the length cannot be determined.

Step 3: Use peek_filelike_length in the elif isinstance(content, Iterable): case of encode_content(...) -> (headers, stream), switching between either Content-Length: <length> or Transfer-Encoding: chunked