fix(async-stripe): Fix header name, extract it to const

http crate does not allow uppercase characters in header name and it was
causing crashes when hitting thing branch with error "index out of
bounds."

The error should happen at compile time, but for some reason the
HeaderName::from_static was executed at runtime, so this commit forces
it to be executed at compile time by extracing it into the const.

Signed-off-by: Robin Ilyk <[email protected]>

Author: robko23

Diff for: async-stripe/src/hyper/client.rs

@@ -109,7 +109,9 @@ impl Client {
         let mut last_error = StripeError::ClientError("invalid strategy".into());
 
         if let Some(key) = strategy.get_key() {
-            req_builder = req_builder.header(HeaderName::from_static("Idempotency-Key"), key);
+            const HEADER_NAME: HeaderName = HeaderName::from_static("idempotency-key");
+            assert!(!key.is_empty(), "idempotency key is empty");
+            req_builder = req_builder.header(HEADER_NAME, key);
         }
 
         loop {