Stringify the keys in SessionHash#merge

[leifg]

As mentioned in #427 the merge! method does not take care of stringifying the keys. That leads to errors if you add symbol (or other non-string) keys as the values will never be read as the fetch converts everything to a string:

def [](key)
    load_for_read!
    super(key.to_s)
end

[travisbot]

This pull request passes (merged f0b47d0 into ea7ed1a).

[raggi]

Apologies for not just merging, duped this change in 6b115c5, as i was looking at the issue not the PR. Avoid this by opening one :)

Thanks.