Clarify OpenAPI 3 coerce_form_params behavior

[ydah]

Fixes #429

• clarify README wording for OpenAPI 3 coerce_form_params behavior
• add a regression test showing that the option coerces JSON request bodies
• update the OpenAPI 3 driver comment to match the implementation

[geemus]

LGTM, definitely appreciate the docs being accurate.

In the future we might consider adding a new flag with a clearer/more obvious name that reflects the new usage/expectations and just keeping this flag for backwards compatibility. That being said, that might also be confusing as it wouldn't really reflect pre openapi 3 behavior. Maybe it should be two flags? I don't know, this seems like the right thing to do for now, but maybe we can revisit at some point.