Handle automatic redirect after switching to hash based history

[robertsLando]

Ref: #3267

[kpine]

This seems to have only been fixed in a very limited use case. Accessing an old URL can still fail with 500 Internal Server Error:

Oct 04 14:05:13 zui[1573243]: TypeError: Cannot read properties of undefined (reading 'replace')
Oct 04 14:05:13 zui[1573243]:     at /usr/src/app/server/app.js:448:51
Oct 04 14:05:13 zui[1573243]:     at Layer.handle [as handle_request] (/usr/src/app/node_modules/express/lib/router/layer.js:95:5)
Oct 04 14:05:13 zui[1573243]:     at trim_prefix (/usr/src/app/node_modules/express/lib/router/index.js:328:13)
Oct 04 14:05:13 zui[1573243]:     at /usr/src/app/node_modules/express/lib/router/index.js:286:9
Oct 04 14:05:13 zui[1573243]:     at Function.process_params (/usr/src/app/node_modules/express/lib/router/index.js:346:12)
Oct 04 14:05:13 zui[1573243]:     at next (/usr/src/app/node_modules/express/lib/router/index.js:280:10)
Oct 04 14:05:13 zui[1573243]:     at /usr/src/app/node_modules/connect-history-api-fallback/lib/index.js:85:5
Oct 04 14:05:13 zui[1573243]:     at Layer.handle [as handle_request] (/usr/src/app/node_modules/express/lib/router/layer.js:95:5)
Oct 04 14:05:13 zui[1573243]:     at trim_prefix (/usr/src/app/node_modules/express/lib/router/index.js:328:13)
Oct 04 14:05:13 zui[1573243]:     at /usr/src/app/node_modules/express/lib/router/index.js:286:9

The code seems to assume a request will always contain the X-External-Path header, which isn't the case if you a) aren't using a proxy, or b) using a proxy but not using a subpath.

[robertsLando]

@kpine fixed on master. I originally made tests without proxy and it worked, then I added the header check too with the proxy and forgot to test without again. Thanks for the feedback, test master if/when you can

[kpine]

LGTM, thanks!