Not allow hash syntax setting

I was attempting to set an attribute on a Her model object using the hash syntax and came across a strange error
```
subject[:OilService] = "bar" 
# => ArgumentError: wrong number of arguments (2 for 1) ... /lib/her/model/attributes.rb:193:in `block (2 levels) in attributes'
```
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.