bump indexmap to 2.0.0

As raised in #39, in some scenarios the `std` detection in `indexmap` causes
compilation failure due to the changing generic parameters of `IndexMap`.
The 2.0.0 series changes the `std` detection behavior.

Cargo.toml

@@ -2,7 +2,7 @@
 authors = ["Patrick Fernie <[email protected]>"]
 description = "Implementation of Cookie storage and retrieval"
 name = "cookie_store"
-version = "0.20.0" # managed by release.sh
+version = "0.21.0-pre" # managed by release.sh
 edition = "2018"
 repository = "https://github.com/pfernie/cookie_store"
 documentation = "https://docs.rs/cookie_store"
@@ -36,7 +36,7 @@ serde_json = "1.0.87"
 time = "0.3.16"
 url = "2.3.1"
 
-indexmap = { version = "1.9.1", optional = true }
+indexmap = { version = "2.0.0", optional = true }
 
 [dependencies.cookie]
 features = ["percent-encode"]