Doesn't support content type application/problem+json

[DanielXu77]

Hi,

It appears content-type "application/problem+json" is not supported by openapi3filter.ResponseValidationInput

I got this error message when trying to verify a response with status code 400 and content type "application/problem+json"
"failed to decode response body: unsupported content type "application/problem+json""

[DanielXu77]

Changes like this should work https://github.com/getkin/kin-openapi/pull/275/files