fix: lowercase all headers in request serialization

The HTTP request class doesn't have any implementations to insure
case-insensitivity of the http headers. So we need to be mindful
when populating these headers. Otherwise, the reqeust signature
will be messed up. This change will ensure the protocol-specific
default headers like content-type will be overriden by the serialized
header if exists.

For other headers added through middleware stack either by
customization or users, it wouldn't affect signing or sending as long
as the request doesn't contain same header names in different casing.
All the internal headers will be consistent. But users should be
careful when they are adding their own headers.

We don't add middleware to lowercase all headers to prevent
alternating the users' customizations.

Ref: aws/aws-sdk-js-v3#1800

Author: AllanZhengYP

smithy-typescript-codegen/src/main/java/software/amazon/smithy/typescript/codegen/integration/HttpBindingProtocolGenerator.java

@@ -365,7 +365,7 @@ private void writeHeaders(
                         String headerValue = getInputValue(context, binding.getLocation(), memberLocation + "!",
                                 binding.getMember(), target);
                         writer.write("...isSerializableHeaderValue($L) && { $S: $L },",
-                                memberLocation, binding.getLocationName(), headerValue);
+                                memberLocation, binding.getLocationName().toLowerCase(Locale.US), headerValue);
                     }
 
                     // Handle assembling prefix headers.
@@ -382,7 +382,8 @@ private void writeHeaders(
                                         String headerValue = getInputValue(context, binding.getLocation(),
                                                 memberLocation + "![suffix]", binding.getMember(), target);
                                         // Append the prefix to key.
-                                        writer.write("acc[$S + suffix] = $L;", binding.getLocationName(), headerValue);
+                                        writer.write("acc[$S + suffix] = $L;",
+                                                binding.getLocationName().toLowerCase(Locale.US), headerValue);
                                         writer.write("return acc;");
                                     });
                             }