fix: allowedMethods() should not fail if more than one router is defined

[lbesson-della]

Fixes #182

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[lukpsaxo]

We have this issue too, would be great to get merged

[zhaoyuqiqi]

You can use “koa-router”, it works

[lbesson-della]

The npm package "koa-router" is on version 12.0.1

This bug concerns version 13.0.0 (which is the latest version published for npm package @koa/router)

[mnahkies]

We just ran into issues with this as well.

In our case we hit the issue when a 404 occurred due to an incorrect url being constructed by a client, but instead of a helpful 404 response/log we got an internal server error with the Cannot read properties of undefined (reading 'length') message from above (note: the stack trace was also hopeless, didn't mention koa router at all).

This made it take longer to diagnose the actual issue than it would've otherwise.

It would be great to get this merged - I've tested the fix from this PR on our issue, and it appears to fix it.