make HEAD/GET method rewrite configurable

[azjezz]

closes #15

[jjergus]

Seems reasonable but Fred has stronger opinions about this so I'll leave it to him. Some potential minor concerns:

• if we later decided on a different solution and removed the flag, it would break backwards compatibility so we should be reasonably sure we want to stick to this solution
• boolean flags can be confusing (as part of a public API, that is -- they're fine in private methods etc.), for someone reading the code it's impossible to know what new SomeClass(false) means, consider replacing that with either
  • a method call: (new SomeClass())->disableHeadRewrite()->...
  • private constructor + pair of static "factory" methods: SomeClass::withHeadRewrite()->... and SomeClass::withoutHeadRewrite()->...

[fredemmott]

I'm fine with the intent of this diff, but not the bool argument; I'm fine with a shape argument, or the instance methods.

The problem with static factory methods is it's not scalable if we want to add other options in the future.

[azjezz]

updated to use options shape 👍

[fredemmott]

Perhaps ?BaseRouterOptions, and make all the fields optional?

This will be needed when a second option is added - it doesn't really matter now, but doing it would make it clearer what the 'right way' is to add one.

This would also need something like:

$this->options = shape(
  'use_get_responder_for_head' => $options['use_get_responder_for_head'] ?? true
);