feat(signature-v4): unsign x-amz-user-agent header (aws#928)

`x-amz-user-agent` is equivalent to `user-agent` header but used only in browsers. If we don't sign `user-agent`, we shouldn't sign `x-amz-user-agent` either.

Author: AllanZhengYP

packages/signature-v4/src/constants.ts

@@ -30,6 +30,7 @@ export const ALWAYS_UNSIGNABLE_HEADERS = {
   "transfer-encoding": true,
   upgrade: true,
   "user-agent": true,
+  "x-amz-user-agent": true,
   "x-amzn-trace-id": true
 };