chore: define headers at declaration

Author: trivikr

codegen/smithy-aws-typescript-codegen/src/main/java/software/amazon/smithy/aws/typescript/codegen/AwsProtocolUtils.java

@@ -59,7 +59,7 @@ static void generateUnsignedPayloadSigV4Header(GenerationContext context, Operat
         operation.getTrait(UnsignedPayloadTrait.class)
                 .filter(trait -> trait.getValues().contains("aws.v4"))
                 .ifPresent(trait -> {
-                    writer.write("headers['x-amz-content-sha256'] = 'UNSIGNED_PAYLOAD';");
+                    writer.write("'x-amz-content-sha256': 'UNSIGNED_PAYLOAD',");
                 });
     }
 

codegen/smithy-aws-typescript-codegen/src/main/java/software/amazon/smithy/aws/typescript/codegen/JsonRpcProtocolGenerator.java

@@ -84,7 +84,7 @@ protected void writeDefaultHeaders(GenerationContext context, OperationShape ope
         // separated by a '.' character, for the target header.
         TypeScriptWriter writer = context.getWriter();
         String target = context.getService().getId().getName() + "." + operation.getId().getName();
-        writer.write("headers['X-Amz-Target'] = $S;", target);
+        writer.write("'X-Amz-Target': $S,", target);
     }
 
     @Override