Properly support maps in Ruby protoc and another bugfix.

Previously, we supported map fields in the Ruby DSL. However, we never
connected the final link in the chain and generated `map` DSL commands
for map fields in `.proto` files. My apologies -- I had been testing
with the DSL directly so I missed this.

Also fixed a handlerdata-setup-infinite-loop when a map value field's
type is its containing message.