Using Dict for headers precludes repeated headers.

[samoconnor]

Following on from #102 ...

A remaining minor concern is that the standard allows multiple repeated headers with the same name, and requires that their order is preserved. See https://stackoverflow.com/a/4371395

Perhaps Vector{Pair} should be allowed.

We could just allow any headers where [String(k) => String(v) for (k, v) in headers] is valid?
e.g.

function f(foo, bar, headers::Vector{Pair{String,String}})
    ...
end

function f(foo, bar, headers) = f(foo, bar, [String(k) => String(v) for (k, v) in headers]