fix: ensure exists works with array of values

[iamstuartwilson]

Overview

When working with this lib I noticed that the exist method was double-wrapping an array of keys due to the way getKeysVarArgs was merging args. This adds a fix and test case to ensure users can use all of the following invocations:

client.exists('key', cb)
client.exists('key', 'key2', cb)
client.exists(['key', 'key2'], cb)

[iamstuartwilson]

@kamiljano We have a workaround for this by shimming the exposed exists methods in our tests, but it would be great if you have time to review this PR. Cheers!