unify absent of HTTP parameter and empty strings

Given:

``` java
{
  post("/path", req -> req.param("foo").toOptional());
}
```

A call to `POST /path` prints `Optional.empty`

While a call to `POST /path?foo=` prints `""` (empty string)

Next release will handle absent or empty string parameter as optional parameter, so both example will print `Optional.empty`
