Setting a new WebProxy still uses the old proxy

- [x ] I have checked that the [SDK documentation][sdk-docs] and [API documentation][api-docs] doesn't solve my issue

### Description of the Issue
Once you set a WebProxy and attempt authentication, setting a new proxy does not take effect. The code seems to still be using the old proxy.

Setting Proxy Address: proxyabc
Error: One or more errors occurred. An error occurred while sending the request. The remote name could not be resolved: 'proxyabc'.
Setting Proxy Address: proxyxyz
Error: One or more errors occurred. An error occurred while sending the request. The remote name could not be resolved: 'proxyabc'.

### Versions Used
.NET 4.51:

### Steps to Reproduce

### Error Message, Including Stack Trace

[sdk-docs]: ./README.md
[api-docs]: https://developer.box.com/docs

Start by reproducing the issue through the WebProxy setting and authentication request path described in the report. Verify that changing from proxyabc to proxyxyz causes the subsequent request to use the new proxy; done means the old proxy is no longer used after the WebProxy is replaced.

Written by the indexing model from the issue text.