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/http/http_connection.hpp b/http/http_connection.hpp
index d061973..6494586 100644
--- a/http/http_connection.hpp
+++ b/http/http_connection.hpp
@@ -35,7 +35,8 @@
 {
     json_html_util::dumpHtml(res.body(), res.jsonValue);
 
-    res.addHeader("Content-Type", "text/html;charset=UTF-8");
+    res.addHeader(boost::beast::http::field::content_type,
+                  "text/html;charset=UTF-8");
 }
 
 static int connectionCount = 0;