Not allow hash syntax setting

[boie0025]

I was attempting to set an attribute on a Her model object using the hash syntax and came across a strange error

> subject[:some_attribute] = "bar" 
# => ArgumentError: wrong number of arguments (2 for 1) ... /lib/her/model/attributes.rb:193

Investigation revealed that Her was interpreting [] as a hash key, therefore the expected hash key was used as the value for the key []. This PR is a quick change that stops Her from using [] as a hash key with a setter.