Allow in newer versions doesn't seem to be backwards compatible?

#### Support plan

* *is this issue currently blocking your project?* (yes/no): no
* *is this issue affecting a production system?* (yes/no): no

#### Context

* *node version*: v14.17.5
* *module version*: 17.4.2 (tried other versions also)
* *environment* (e.g. node, browser, native): node
* *used with* (e.g. hapi application, another framework, standalone, ...): N/A
* *any other relevant information*: N/A

#### How can we help?

`joi.string().allow('HEAD').hex().min(6).max(40).lowercase().required()` does not allow for the string `HEAD`, which is explicitly allowed. Can someone explain what I am doing wrong?

Reproduce the reported schema with Joi 17.4.2 on Node 14.17.5, then inspect how allow('HEAD') interacts with hex(), min(), max(), lowercase(), and required(). No files or tests are named in the issue; done means determining whether this is intended behavior or a regression and, if it is a bug, identifying a focused test case.

Written by the indexing model from the issue text.