fix #1509: Wrong HTTP status code with jTcpdfResponse

In some hosting, the http status code may be 404. We should
force the http status code, to send all http headers.

New $overwrite parameter value for jResponse::addHttpHeader():
-1 means to not overwrite the existing value but to keep it
and to add the new value.
Build: more help in set_new_version.sh
Fixed a unit tests
Fix a regression made by the fix of ticket #1503
Release Jelix 1.2.12
Bumped version to 1.2.13pre.SERIAL