fixed stream responders lifetimes

[Arteneko]

The current implementation of the stream responders have a very restricted lifetime which doesn't allow using them, and this lifetime restriction doesn't appear on the other responder implementations.

A short example of the current implementation's usage:

#[derive(Responder)]
pub enum Woof {
	Woof(ReaderStream<One<&'static [u8]>>),
}

The error generated is the following:

error: lifetime may not live long enough
  --> src/routes/catchers.rs:56:10
   |
56 | #[derive(Responder)]
   |          ^^^^^^^^^
   |          |
   |          has type `&rocket::Request<'1>`
   |          lifetime `'o` defined here
   |          method was supposed to return data with lifetime `'o` but it is returning data with lifetime `'1`
   |
   = note: this error originates in the derive macro `Responder` (in Nightly builds, run with -Z macro-backtrace for more info)

help: add bound `'r: 'o`

The generated code from the derive shows that the rocket::response::Result lifetime is the same as the rocket::request::Request and can't live longer than that.

impl<'r, 'o: 'r> ::rocket::response::Responder<'r, 'o> for Woof {
	fn respond_to(
		self,
		__req: &'r ::rocket::request::Request<'_>,
	) -> ::rocket::response::Result<'o> {
		match self {
			Woof::Woof(__0) => {
				let mut __res =
					<ReaderStream<One<&'_ [u8]>> as ::rocket::response::Responder>::respond_to(
						__0, __req,
					)?;
				::std::result::Result::Ok(__res)
			}
		}
	}
}

Fixing the lifetimes (in the same pattern as what's been done on e.g. &str) solves the issue.

i have tested the fix on ReaderStream as it's the one i'm using in my project and applied the same fix over to the other stream implementations but as i don't understand them as much i didn't properly test them on this.
Note: My project's rocket version is 0.5.1, that's where a friend and i initially found the issue leading to this fix.