Invalid "agent" request property when retrying request

[krzysztof-grzybek]

Describe the bug

When using DefaultRequestExecutor with HTTPS_PROXY env variable being set, retrying request fails with error The "options.agent" property must be one of Agent-like Object, undefined, or false. Received an instance of Object, which is coming from node http(s) library.

The cause.

1. We're setting "agent" to an object of type HttpsProxyAgent - https://github.com/okta/okta-sdk-nodejs/blob/master/src/http.js#L64
2. When retrying request, we deepCopy previous request - https://github.com/okta/okta-sdk-nodejs/blob/master/src/default-request-executor.js#L36
3. After deep copying object, "agent" is no longer an instance of HttpsProxyAgent, and that's the cause why node http(s) library throws an error.

Reproduction Steps?

1. Run library with HTTPS_PROXY being set.
2. Trigger request retry (e.g. API limit error)
3. Error The "options.agent" property must be one of Agent-like Object, undefined, or false. Received an instance of Object is thrown.

SDK Versions

7.0.1

Additional Information

No response