Allow repeated query parameters

This fixes https://github.com/dart-gde/discovery_api_dart_client_generator/issues/48

@nikgraf - can you maybe checkout my fork and see if it works? Methods with repeated query parameters now accept a List of values instead of a single value and the request is constructed accordingly.

`resource.method(key: [value1, value2, value3])` will create the request `method_url?key=value1&key=value2&key=value3`

@financeCoding - this won't effect existing Google client libraries since I haven't seen this special use-case in the API docs before, so no need to make a new force update of them.