`RequestBuilder::headers` will always overwrite any existing header with
the same name, but will now correctly append extra values from the *new*
header map.
Closes#407
This means that `build` cannot possibly panic anymore due to being
called multiple times. This is a breaking change as it breaks the
behaviour of builder methods called without assigning to a new variable
or chaining. It's rather easy to fix those usages, as they won't
compile anymore and can be fixed by assigning a result.
Additionally, this change reduces the size of `RequestBuilder`,
although this likely isn't all that meaningful, as usually there
is no reason to store builders in structures.
