allow vendor content types

[bluebrown]

Hi, I am using vendor content type like application/vnd.org.v1+json. Because of this, response validation failed with unsupported content type.

Is there a way to tell it to decode it as json?

[bluebrown]

ok, I found out that I can use openapi3filter.RegisterBodyDecoder. Its just a bit inconvinient that the default included decoders are private, so I had to copy the json one to provide it as argument.

Maybe the default decoders could be public?

[nickajacks1]

@bluebrown looks like this can be closed due to #896