title: Echo Groups not working with OpenAPI generated code using oapi-codegen
body: <p>I am using <a href="https://github.com/deepmap/oapi-codegen" rel="nofollow noreferrer">oapi-codegen</a> to generate my server code and Echo Labstack as the server.
  When I pass a <code>Group</code> instance to <code>Openapi.RegisterHandlers</code> instead of an <code>Echo</code> instance, I always get a 400 error with <code>{&quot;message&quot;:&quot;no matching operation was found&quot;}</code> for any request in that group:</p>
  <pre><code>    swagger, err := Openapi.GetSwagger()
      if err != nil {
          fmt.Fprintf(os.Stderr, &quot;Error loading swagger spec\n: %s&quot;, err)
          os.Exit(1)
      }
  
      // Use oapi validation middleware to check all requests against the
      // OpenAPI schema.
      g := e.Group(&quot;/api&quot;, middleware.OapiRequestValidator(swagger))
      Openapi.RegisterHandlers(g, &amp;MyApi{})
  </code></pre>
  <p>If send request <code>/api/foo</code>, where <code>foo</code> is an API endpoint defined in the generated server code, I get a 400 error. If I do <code>/api/&lt;some undefined api&gt;</code> I also get 400. If I do send a request for <code>/baz</code>, I get 404 as expected, since that isn't a defined route. If I don't pass a prefix to <code>Group()</code>, I get a 400 error for every request. I get the same behavior if I use <code>RegisterHandlersWithBaseURL()</code></p>