Cast back what we get when updating to symbols so that we get consistent results

[mat813]

In Rails, the parameters we get from the web are all strings, but bitmask-attribute uses symbols. passing it strings is not a problem, as it gets it right, but it keeps the string in it's cache and it's a mess :

>> c.code
[
    [0] :power
]
>> c.code?(:power)
true
>> c.code?('power')
false
>> c.update_attributes("code" => ["power"])
true
>> c.code?(:power)
false
>> c.code?('power')
true
>> c.code
[
    [0] "power"
]

With this commit, it does :

>> c.code
[
    [0] :power
]
>> c.update_attributes("code" => ["power"])
true
>> c.code?(:power)
true
>> c.code 
[
    [0] :power
]