object-store: http-store fails when url doesn't have last-modified header on 0.7.0

[universalmind303]

Describe the bug
prior to 0.7.0 of object_store, you could fetch from urls that didn't have last-modified header, after 0.7.0 i get the error

 Unable to extract metadata from headers: Last-Modified Header missing from response

To Reproduce
cargo.toml

[package]
name = "object-store-mre"
version = "0.1.0"
edition = "2021"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]
object_store = { version = "0.7.0", features = ["http", "aws", "azure", "gcp"] }
tokio = { version = "1.32.0", features = ["rt-multi-thread"] }

main.rs

use object_store::http::HttpBuilder;
use object_store::path::Path;
use object_store::ObjectStore;

#[tokio::main]
async fn main() {
    let http = HttpBuilder::new().with_url("https://raw.githubusercontent.com/GlareDB/glaredb/main/testdata/sqllogictests_datasources_common/data/bikeshare_stations.ndjson");
    let store = http.build().unwrap();
    let path = Path::default();

    let res = store.get(&path).await.unwrap();
    println!("{:?}", res);
}

Expected behavior
works like it did in previous versions

Additional context

[tustvold]

label_issue.py automatically added labels {'object-store'} from #4834