This fixes an issue where query parameters that are objects with additional properties cannot be parsed into their values. For example, a query parameter like tags=foo,bar,bim,baz should end up becoming:

map[string]string{
  "foo": "bar",
  "bim": "baz",
}

This adheres to the spec for paramters of style=form and explode=false.

Improves the problem described in oapi-codegen/oapi-codegen#698 for empty schemas with additional properties, but doesn't address the issue of a non-empty schema with additional properties.