Allow setting the request method for Turbo Frame requests to avoid 414 Long URL errors

[ghiculescu]

Currently the request that a Turbo Frame makes to load a page is hardcoded to a GET request. We came across an edge case where HTTP 414s were being thrown due to a very large request URL, in a complex form that heavily uses frames with URLs to hold form state.

The general approach to handling 414s is to switch to a POST. This PR adds the ability to declaratively do that on the frame element:

<turbo-frame id="stuff" src="/my/url" method="post" params='{"required":true,"type":"string"}'></turbo-frame>

You need to provide:

• method= "post"
• params = Something that URLSearchParams can interpret: stringified JSON, or form params.

This will make a POST to /my/url, with the provided params in the POST body.