Add append_header to RequestBuilder or make header types fully public

[laundmo]

Currently, if i want to use RequestBuilder with multiple values for the same header, i have to construct a weird iterator using HeaderValue::from_str since the necessary tools to do this are missing.

I cannot:

• simply loop and call append_header, since RequestBuilder doesn't have this
• construct a Headers, since its private and i can't pass it to RequestBuilder
• construct and HeaderValues for RequestBuilder::header since its private

The only way i want to have multiple header values with RequestBuilder, i need to do this:

builder = builder.header(
    name,
    vec!["a", "b"]
        .iter()
        .map(|f| HeaderValue::from_str(f).unwrap().collect()),
);

which is quite clunky