panic in accumulator if nil is passed as the third parameter to Add

[gotyaoi]

I was testing the changes I had made for the config directory support when I came across this. The changes here: 6977119#diff-2df0b9f3bc0da3ee13966a69e386ff84R145 will panic if Add is passed nil as the tags parameter. A quick grep shows this only happens in the system plugin, the puppetagent plugin and as an example in the CONTRIBUTING.md file.

Edit: my grep was insufficient. This also occurs in the memory plugin, as it passes map[string]string(nil).

[sparrc]

I'm working on overhauling the accumulator and already have a fix for this in that branch

[gotyaoi]

Ok, cool.

[sparrc]

I'll leave this case open until I actually get that pushed, thanks for the report @gotyaoi!