Add json to the known content types

Recently the odata endpoint has been implemented as a static file.
This adds the appropriate content type to the map, so that we can
return the appropriate header.

Fixes the error:
Cannot determine content-type for
"/usr/share/www/redfish/v1/odata/index.json" with extension .json

Change-Id: Ia6d55c9644c344a2d1957b878bfc25a994e8678b
diff --git a/include/webassets.hpp b/include/webassets.hpp
index 161fc50..816d543 100644
--- a/include/webassets.hpp
+++ b/include/webassets.hpp
@@ -41,6 +41,7 @@
            {".xml", "application/xml"},
            {".jpg", "image/jpeg"},
            {".jpeg", "image/jpeg"},
+           {".json", "application/json"},
            // dev tools don't care about map type, setting to json causes
            // browser to show as text
            // https://stackoverflow.com/questions/19911929/what-mime-type-should-i-use-for-javascript-source-map-files