Throw error if 'allowReserved' is set to true

[charjr]

https://github.com/OAI/OpenAPI-Specification/blob/v3.0.4-dev/versions/3.0.4.md#fixed-fields-for-use-with-schema

When this is true, parameter values are serialized using reserved expansion, as defined by RFC6570, which allows RFC3986's reserved character set, as well as percent-encoded triples, to pass through unchanged, while still percent-encoding all other disallowed characters (including % outside of percent-encoded triples). Applications are still responsible for percent-encoding reserved characters that are not allowed in the query string ([, ], #), or have a special meaning in application/x-www-form-urlencoded (-, &, +); see Appendices C and E for details. This field only applies to parameters with an in value of query. The default value is false.

We currently do not handle this when parsing query strings, so we should throw a clear exception to mention this.

The reader will need to keep track of this keyword on parameters for this to happen.