[BV-4887] Refactor mismatched names between GET (nick_name) & PUT/POST (nickname). Remove unneeded methods in BadgevilleBerlin::Player class. Revise customize_keys_for_request to always reference symbols, not strings, and then called it in BaseResource#load, not in BaseResource#sanitize_request. Revise RSpecs to show the nick_name= setter does not set nickname without the underscore.