Allow prefixed headers to be overridden by explicit headers in rest protocols (#3225)

Author: mullermp

gems/aws-sdk-core/CHANGELOG.md

@@ -1,6 +1,8 @@
 Unreleased Changes
 ------------------
 
+* Issue - Allow explicit modeled headers to override prefixed headers for `rest` protocols. 
+
 3.222.0 (2025-03-27)
 ------------------
 

gems/aws-sdk-core/lib/aws-sdk-core/rest/request/headers.rb

@@ -68,7 +68,7 @@ def list(headers, ref, values)
         def apply_header_map(headers, ref, values)
           prefix = ref.location_name || ''
           values.each_pair do |name, value|
-            headers["#{prefix}#{name}"] = value.to_s
+            headers["#{prefix}#{name}"] ||= value.to_s
           end
         end