options.sourceOriginPattern.test is not a function

I am using `@ampproject/toolbox-cors` for setting cors for on express server.
I get following error
```
{
"errorType": "Runtime.UnhandledPromiseRejection",
"errorMessage": "TypeError: options.sourceOriginPattern.test is not a function",
"reason": {
"errorType": "TypeError",
"errorMessage": "options.sourceOriginPattern.test is not a function",
"stack": [
"TypeError: options.sourceOriginPattern.test is not a function",
" at normalizedHeaders (/var/task/node_modules/@ampproject/toolbox-cors/lib/cors.js:65:69)",
" at Layer.handle [as handle_request] (/var/task/node_modules/express/lib/router/layer.js:95:5)",
" at trim_prefix (/var/task/node_modules/express/lib/router/index.js:317:13)",
" at /var/task/node_modules/express/lib/router/index.js:284:7",
" at Function.process_params (/var/task/node_modules/express/lib/router/index.js:335:12)",
" at next (/var/task/node_modules/express/lib/router/index.js:275:10)",
" at apiGatewayEventParser (/var/task/node_modules/aws-serverless-express/src/middleware.js:22:3)",
" at Layer.handle [as handle_request] (/var/task/node_modules/express/lib/router/layer.js:95:5)",
" at trim_prefix (/var/task/node_modules/express/lib/router/index.js:317:13)",
" at /var/task/node_modules/express/lib/router/index.js:284:7"
]
},
"promise": {},
"stack": [
"Runtime.UnhandledPromiseRejection: TypeError: options.sourceOriginPattern.test is not a function",
" at process. (/var/runtime/index.js:35:15)",
" at process.emit (events.js:310:20)",
" at processPromiseRejections (internal/process/promises.js:209:33)",
" at processTicksAndRejections (internal/process/task_queues.js:98:32)"
]
}
```
If I don't use `sourceOriginPattern` then it works fine. When I use it then I get error from above.
```
app.use(ampCors({
verbose: true,
sourceOriginPattern: "/https:\/\/apoorv\.blog$/",
verifyOrigin: false
}))
```

Start at lib/cors.js:65, where normalizedHeaders calls options.sourceOriginPattern.test, and compare that entry point with the app.use(ampCors(...)) configuration shown in the issue. Confirm the expected type for sourceOriginPattern and verify that the middleware no longer throws when the documented configuration is used.

Written by the indexing model from the issue text.