add NewClientFromReq method to allow streaming any kind of requests

[radiantspace]

Allows making sse clients from any kind of requests, not just GET.

I needed that for streaming responses from OpenAI like this:

req, err := http.NewRequestWithContext(ctx, http.MethodPost, "https://api.openai.com/v1/chat/completions", bytes.NewBuffer(body))
if err != nil {
    return nil, err
}
req.Header.Set("Content-Type", "application/json")
req.Header.Set("Authorization", "Bearer "+a.authToken)

client := sse.NewClientFromReq(req)

...

Closes #153

[wuhan005]

Suggested change

	if c.Request != nil {
	} else {
	if c.Request == nil {