Tighten Solve's check for tautologies (issue #208)

SymPy returns True if an equation is tautological. Since [1] == [True]
in Python, Mathics would accidentally treat an equation with one
result equal to 1 as a tautology. Fix this by using "is True" to check
for True.
Add test for issue #208
Merge pull request #216 from bnjones/master

Tighten Solve's check for tautologies (issue #208)