Fixed minor dummification issue.

Nothing too big, just looking at the namespaces used for eval rather than the
names of modules passed in.
Added dummify flag to lambdify.

Replaced logic in lambdify which attempted to determine if dummification should
or should not happen, with a flag (default True) for dummification. The idea is
if someone wants to read the output and are already supplying 'sympy' as a
module, they shouldn't have a problem supplying dummify=False as an argument.
Corrections regarding PR 2428

Lambdify has no dummify option - it is default. Dummification only happens for
functions, not symbols.
Merge pull request #2428 from gilbertgede/fix_lambdify

Fixed minor dummification issue.