Pattern + None returns Reaction Pattern, + tests

d-fan introduced changes to allow MonomerPatterns, ComplexPatterns, and
ReactionPatterns to be added to None. However, the implementation returned the
initial Monomer/ComplexPattern rather than a ReactionPattern. Here I convert
MonomerPattern and ComplexPatterns added in this way to ReactionPatterns using
as_reaction_pattern, and add the appropriate tests. Note that ReactionPattern +
None can return self, rather than as_reaction_pattern(self), because, well,
it's already a ReactionPattern.
Reverted doctest ellipsis for Solver

See @jmuhlich's comment on the lolab pull request at
https://github.com/pysb/pysb/pull/87. He fixed the root cause in commit
08aeae9b17b so this change can be reverted.