Upgrade deadpool dependency

[emmiegit]

As described in #109, this issue upgrades the deadpool dependency, which through a chain of dependencies relies on an outdated version of nom which has code that rustc now emits a deprecation warning for:

    Finished dev [unoptimized + debuginfo] target(s) in 4m 49s
warning: the following packages contain code that will be rejected by a future version of Rust: nom v5.1.2
note: to see what the problems were, use the option `--future-incompat-report`, or run `cargo report future-incompatibilities --id 35`

The other dependencies in this chain are already upgraded, so fixing the use here in http-client will result in downstream users no longer having this rustc warning appear.