Don't change param names when generating Feign interfaces

At the moment param names are changed from snake_case to camelCase when generating Feign interfaces.

So when generating client code for this spec:

parameters: [
    {
        name: "grant_type",
        in: "formData",
        description: "client_credentials",
        required: true,
        type: "string",
        default: "client_credentials"
    }
]

your body ends up like this:

{
grandtType: "XXXX"
}
when it actually should look like this:

{
grant_type: "XXXX"
}

Swagger-codegen version

2.2.2

Start by locating the Feign interface generator and the parameter-name conversion used for formData parameters. Reproduce the provided specification and verify that the generated request body preserves grant_type instead of changing it to camelCase; add or update the relevant generator coverage if one is found.