Bump serde_urlencoded to 0.7.1 (#1457)

serde_urlencoded 0.7.0 has an incorrect serde version dependency.
This causes issues in `cargo update -Z minimal-versions` builds.
It's not noticeable while testing in CI because cookie_store depends
on serde 1.0.126, however because this is an optional dependency
there is a possibility that a project using reqwest won't enable
it.

Cargo.toml

@@ -80,7 +80,7 @@ http = "0.2"
 url = "2.2"
 bytes = "1.0"
 serde = "1.0"
-serde_urlencoded = "0.7"
+serde_urlencoded = "0.7.1"
 
 # Optional deps...