It is impossible to reject malformed optional form fields with anyParams

[at-nap]

When using anyParams, it is impossible to reject malformed optional form fields; it unconditionally falls back to the query param, which is missing but optional, and therefor valid.

I.e:

  // Passes as expected
  "anyParam directive" should "reject malformed optional query parameters" in {
    Post("/?foo=bar", FormData(Seq.empty)) ~> anyParams('foo.as[Option[Int]]) {
      foo => complete(s"foo: $foo")
    } ~> check {
      rejections.head shouldBe a [MalformedQueryParamRejection]
    }
  }

  // Fails!
  it should "reject malformed optional form fields" in {
    Post("/", FormData(Seq("foo" -> "bar"))) ~> anyParams('foo.as[Option[Int]]) {
      foo => complete(s"foo: $foo")
    } ~> check {
      rejections.head shouldBe a [MalformedFormFieldRejection]
    }
  }

The second test fails:

...
[info] - should reject malformed optional form fields *** FAILED *** (8 milliseconds)
[info]   Request was not rejected, response was HttpResponse(200 OK,HttpEntity(text/plain; charset=UTF-8,foo: None),List(),HTTP/1.1)
...

This behaviour is unexpected. It seems anyParams ought to fall back to query params only on missing form fields. (Or anyParams ought to be renamed any*Valid*Params, if you get my drift.)