Bug 4793

Before 5.12, il was possible to place a reverse-proxy before Silverpeas to open a secure channel: 
client <-- HTTPS --> reverse-proxy <-- HTTP --> Silverpeas
With the first releases of 5.12, this feature was no more possible: the secure channel has had to be continued up silverpeas itself : <-- HTTPS --> reverse-proxy <-- HTTPS --> Silverpeas
This PR fixes this situation by restoring the previous behaviour. Now, when Silverpeas is after a reverse-proxy that handles HTTPS connections, the parameter server.ssl in org/silverpeas/general.properties has to be set to true.