Remove DEFAULT_HOST (#153)

In a few places we relied on "localhost" as a default if a URL's host
was not set, but I think it's better to error out in these cases.

In general, there are a few places in Unit that assumed there is a
host as part of the URL. I've made that explicit by doing a check
at the beginning of `connect()`. I've also tried to plumb through
the semantics of "host is always present" by changing the parameter
types of some of the functions that use the hostname.

I considered a more thorough way to express this with types - for
instance implementing an `HttpUrl` struct that embeds a `Url`, and
exports most of the same methods, but guarantees that host is always
present. However, that was more invasive than this so I did a smaller
change to start.

src/pool.rs

@@ -8,7 +8,6 @@ use crate::Proxy;
 
 use url::Url;
 
-pub const DEFAULT_HOST: &str = "localhost";
 const DEFAULT_MAX_IDLE_CONNECTIONS: usize = 100;
 const DEFAULT_MAX_IDLE_CONNECTIONS_PER_HOST: usize = 1;