core: Fix the command-line default string

Symptom: When a command line parameter's default value string contains a white space, it is not printed in the help message

Root Cause Analysis: While the construction of the stringstream of a particular data type can be arbitrarily designed, possibly including white spaces, the command-line class either inadvertentely failed to support a string including white spaces or intentionally decided not to support a white character in the default value string. The latter case is a design bug, if intended, because the command line class cannot enforce how a data type's string may be constructed.

Fix: Take the whole string the stringstream constructs.