Fix Scalatra Test so client can send multiple headers of the same name

I want to be able to do the following from my test code:
```
        post("/someservice/" + id, data,
             Seq("Content-Type" -> "application/json",
                 "x-name"  -> "foo",
                 "x-name"  -> "bar",
                 "x-name"  -> "none")) {
            status should equal (201)
        }
```
Currently, the Scalatra Test library defines post() and get() only to take Map() as headers. This Pull Request changes that to take Seq().