Feature request: allow configure default "Accept" header

[weilai-irl]

In OpenAPI specification 3.0 or later, it's not possible to define an "Accept" header for all API requests, rather the value of the Accept header is defined in each function. As a result, when making a web service call from systems complying with OpenAPI spec, no Accept header is included. This would result in an error straight away.

The solution we come up with is to allow configuring a default Accept header content in the plugin, and use it if (1) Accept header doesn't present, AND (2) the option is enabled, AND (3) a default Accept header value is configured.

A PR has been / will be created. Please consider merging this.

Regards,
Lai