Use boolean strings for cache keys

[pshrmn]

Previously, the cache keys for compiled path-to-regexp patterns was being computed using:

const cacheKey = options.end + options.strict

However, this meant that { end: true, strict: false } created the same key as { end: false, strict: true }.

[mjackson]

Oh man, I thought true + false would be truefalse. Thanks for catching this :)