Fixup unit tests and various serialization bugs

* reverted 5bba51c396f1c10907220eafb25bbf76068d9ec3 as the bug was elsewhere (previous workaround in Socket::get is no more required)
* various bugfixes in Server class
* catch specialized exception (UserException) instead of std::exception in diet_initialize unit test (configuration file not found)
* now all unit tests are green