[erlang-client] Extra commas in generated lists

Description

When an API method has a mix of required and optional parameters the generated code will include spurious commas.

Optional:
 * offset
 * limit
 * sort
 * order
 * format
 * fields

Required:
 * timezone

Generated code snippet

    QS = lists:flatten([{<<"timezone">>, Timezone}])++[{X, maps:get(X, _Optional)} || X <- [, 'offset', 'limit', 'sort', 'order', 'format', 'fields'], maps:is_key(X, _Optional)],

Note, if I change the order of the fields in the swagger json/yaml (define the required field after the optional fields), the comma will appear in the 'required' section instead

Swagger-codegen version

2.3.1

Command line used for generation

java -jar swagger-codegen-cli.jar generate -i /tmp/swagger.json -l erlang-client --additional-properties packageName=test

Suggest a fix/enhancement

This appears to be because the params are handled as a single array, and the {{^first}}, {{/-first}} method for injecting the commas does not work inside a filtered list (required/not required)

(at least according to https://github.com/samskivert/jmustache#-first-and--last)

Maybe splitting the params out into two separate arrays of required and optional would help

Start at the erlang-client generator invoked through swagger-codegen-cli.jar and trace how required and optional parameters are rendered in the generated query-string list. Reproduce the issue with an API method containing both parameter types, then verify that generated Erlang code contains no spurious commas.