Normally a valueless symbol remains valueless and a symbol with a
value keeps having one. But just in case a symbol does get changed
from valueless to having a value, make sure there's a space between
the symbol and the value. And if a symbol gets changed from having a
value to valueless, strip trailing whitespace.
Add corresponding tests.
Also fix the case of a valueless symbol added with the set method,
which would have resulted in attempting to use None as a string. This
only happened with the Python API, not with the command line API.
261742bd59