feat(mock/http/body): without_eos() disables is_end_stream() hints

[cratelyn]

the http_body::Body::is_end_stream() trait method is provided, by
default, such that it always returns false.

- https://docs.rs/http-body/latest/src/http_body/lib.rs.html#58-60

pub trait Body {
    // ...

    /// Returns `true` when the end of stream has been reached.
    ///
    /// An end of stream means that `poll_frame` will return `None`.
    ///
    /// A return value of `false` **does not** guarantee that a value will be
    /// returned from `poll_frame`.
    fn is_end_stream(&self) -> bool {
        false
    }

    // ...
}

we have a MockBody type that allows unit tests to exercise how body
middleware to particular sequences of poll_frame() outcomes. today,
the MockBody type reports the is_end_stream() hint when there are no
more poll values to yield.

this commit introduces a new method, MockBody::without_eos(), which
will instruct the mock body to always return false from
is_end_stream().

this allows test cases to exercise end-of-stream cases in which an inner
Body does not provide a more accurate hint than the provided default.

Signed-off-by: katelyn martin [email protected]