Consistent treatment for strings and symbols in field references

[geshwho]

#105 introduced new behavior that allows us to specify fields as symbols and automatically have the camelcased (e.g. :product_id becomes productId). In order to keep the change backwards compatible, the behavior of Strings remains unchanged. This means that there is inconsistent behavior between strings and symbols.

It would be great it, instead, we also camelize string input and give a global configuration option to either disable camelcase. This behavior would also be more inline with how graphql-ruby behaves.