Use enum overload for field setting

There are two overloads of addHeader, one that takes a string, and one
that takes a boost enum.  For most common headers, boost contains a
string table with all of those entries anyway, so there's no point in
duplicating the strings, and ensures that we don't make trivial
mistakes, like capitalization or - versus underscore that aren't caught
at compile time.

Tested:
This saves a trivial amount (572 bytes) of compressed binary size.

curl --insecure -vvv --user root:0penBmc https://192.168.7.2/redfish/v1

returns < Content-Type: application/json

curl --insecure -vvv -H "Accept: text/html" --user root:0penBmc https://192.168.7.2/redfish/v1

Returns
< Content-Type: text/html;charset=UTF-8

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: I34c198b4f9e219247fcfe719f9b3616d35aea3dc
diff --git a/include/webassets.hpp b/include/webassets.hpp
index 3d25296..cd57eb3 100644
--- a/include/webassets.hpp
+++ b/include/webassets.hpp
@@ -152,13 +152,15 @@
                     const std::shared_ptr<bmcweb::AsyncResp>& asyncResp) {
                 if (contentType != nullptr)
                 {
-                    asyncResp->res.addHeader("Content-Type", contentType);
+                    asyncResp->res.addHeader(
+                        boost::beast::http::field::content_type, contentType);
                 }
 
                 if (contentEncoding != nullptr)
                 {
-                    asyncResp->res.addHeader("Content-Encoding",
-                                             contentEncoding);
+                    asyncResp->res.addHeader(
+                        boost::beast::http::field::content_encoding,
+                        contentEncoding);
                 }
 
                 // res.set_header("Cache-Control", "public, max-age=86400");