Make sure to namespace keys for EVAL/EVALSHA.

*I'd like to get this in ASAP, as this ties in with my work with @bvandenbos on making `resque-scheduler` more failure resistent. Thanks!*

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The call semantics for both these methods are pretty specific, and require their own special case :(

Ex:

  # Both these are valid!
  redis.eval("return KEY[1]", %w[key1], %w[arg1 arg2])
  redis.eval("return KEY[1]", :keys => %w[key1], :argv => %w[arg1 arg2])

Any keys passed to Lua need to retain the correct namespace, which this pull request addresses.