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/ibm/management_console_rest.hpp b/include/ibm/management_console_rest.hpp
index 0d8e287..7396ce6 100644
--- a/include/ibm/management_console_rest.hpp
+++ b/include/ibm/management_console_rest.hpp
@@ -321,7 +321,8 @@
 
     std::string contentDispositionParam =
         "attachment; filename=\"" + fileID + "\"";
-    asyncResp->res.addHeader("Content-Disposition", contentDispositionParam);
+    asyncResp->res.addHeader(boost::beast::http::field::content_disposition,
+                             contentDispositionParam);
     std::string fileData;
     fileData = {std::istreambuf_iterator<char>(readfile),
                 std::istreambuf_iterator<char>()};