fix: handle missing response headers case

[powersjcb]

resolves issue with testing libraries where error.response.headers is undefined and we cannot access the retry-after header.

[powersjcb]

This is currently manifesting with axios-mock-adapter@1.22.0 -> axios-mock-adapter@2.5.1.

I might try to get fix applied to that project too, but raising it here for now since this is a trivial change.

TypeError: Cannot read properties of undefined (reading 'retry-after')
    at retryAfter (node_modules/axios-retry/dist/cjs/index.js:67:145)
    at exponentialDelay (node_modules/axios-retry/dist/cjs/index.js:85:45)

[powersjcb]

Have thoughts about which side we should resolve this on?

ctimmerm/axios-mock-adapter#408