Don't proxy `#eval` as it raises TypeError on nested namespaces
Merge branch 'issue-67/nested-namespace-vs-eval'

Similar to 6d839515e8a3fdc17, in which the private method `exec` command
was resolved instead of Redis::Namespace#method_missing, the private
method `eval` is equally dangerous, especially in nested namespaces.

HT to @rgraff for the report and patch.