proxy implementation in httpclient
First of all, thank you guys so much for putting effort this amazing project. I really enjoy using it and learning how to make the NIM project clean and organized. I was wondering, does the httpclient support regular http/https proxy, just like NIM std httpclient library? I went through the code and I didn't find code can realize this functionality but just want to make sure. 

If not I think it should be easy to implement and I can try to do that then make PR. The only thing worries me that is you have very well defined test cases and I don't have anything on top of my head how to make a robust test case for the http proxy (mainly to find a long lifetime public proxy server). 