Feature/swagger ui behind proxy

[adroffner]

This branch adds a behind_proxy=True keyword parameter to the flask_restplus.api.Api(..., behind_proxy=True) object. Many users need a reverse proxy like nginx to serve the REST API. In my case, I had to place the server inside a kubernetes pod & keep its port number.

I added appropriate unit tests to the test_api.py file. It considers both cases when behind_proxy is either True or False. I assume that SERVER_NAME is set & has a defined port.

OTHER ISSUES: I did not want to fix these in this branch & make it change too much

• I had to pip install wheel to make the invoke dist work, which is not listed in the requirements directory.
• I had to run invoke assets before invoke dist so that the Swagger UI javascript gets into the wheel
• invoke qa shows many mccabe complexity violations (C901 flake8 warnings) that stop the build

The C901 complexity warnings mean no one can just run inv all to complete a build :(

[coveralls]

Coverage increased (+0.007%) to 96.852% when pulling feab0fd on adroffner:feature/swagger-ui-behind-proxy into a8f3582 on noirbizarre:master.

[michaelbukachi]

I really hope this pull request is merged. There's hack I keep copying from project to project just to apps work behind proxies.😂