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.
- A golden-file test that ensures protoc produces known-valid output.
- A Ruby test that loads that golden file and ensures it actually works
with the extension.
This split strategy allows us to test end-to-end without needing to
integrate the Ruby gem build system and the protoc build system. This is
desirable because we do not want a gem build/install to depend on
building protoc, and we do not want building protoc to depend on
building and testing the gem.
