Use prototypes for model methods

[delenius]

Fixes #2044.

[wing328]

@delenius thanks for the PR.

I notice that the commit is by "elenius", which means this won't count as your contribution in https://github.com/swagger-api/swagger-codegen/graphs/contributors.

Do you want to fix it by closing this one and submit a new one with the commit correctly linked to @delenius ?

[delenius]

Good catch, I will do that.

[delenius]

@wing328, I filed PR #2071 to update the petstore example.