Add a config option to disable or change pluralization

[wolfgang42]

I have an API which does not pluralize the names of its models. This PR adds an option to disable pluralization so I can use Devour as my client, and also custom pluralization for other users who might have different pluralization requirements not covered by the default pluralize.

Incidental to this change is an update to an error message. The original problem was that the error message did not reflect the expected pluralization, leading to the confusing error The JSON API response had a type of "Products" but Devour expected the type to be "Products". While I was at it I also changed the message to reflect the actual underlying problem, modifying the code originally added by #10 as the response model was no longer needed for the error message.

[spiderbites]

Hi @wolfgang42 thanks for this PR!

Just wondering if you looked at using the collectionPath option when defining your models, I think this would have been a way for force non-pluralized model names, although with a large number of models this is definitely a cleaner solution.

[wolfgang42]

@spiderbites The problem with that is Devour also tries to singularize the name of the model, so if your models are plural it can't find them. I could try to keep track of which bits needed to be singular and which bits needed to be plural, but it was altogether simpler to turn off pluralization entirely so I can use the name that the API gives it everywhere without having to worry about conversions.