Properly submit the right HTTP method type to rails

So, in some scenarios, Rails will embed the expected action for the route inside the form itself, as a `<input type="hidden" name="_method" value="PATCH">`

Our serializer serializes that, and Rails is happy.

The mistake was in assuming that Rails always places this `_method` input there.  In at least the following scenarios, it does not:

- GET
- POST
- PUT (the surprising one)

This results in cases where your endpoint only responds to `PUT` and we're doing a `POST` with `tg-remote` forms => 404.  I believe this also causes `method="GET"` forms to do a POST atm

I'm betting it's because of browser limitations that it doesn't actually use the native HTTP method type.

Solution:  Use `method` attribute on `form`, and serialize it into the mix *if there is no `_method` there already*

This PR adds example forms of each type + tests for each.  

cc @celsodantas @tylermercier @nsimmons @kurtfunai @patrickdonovan  
and why are we injecting in the form? If it's not in the form, it means that Rails didn't added it there, so I don't see why we should.