HTTP/1.1 Proxy-Connection header field
The Proxy-Connection field is NOT a standard HTTP header field.
It is quite common to find the Proxy-Connection header field in the wild. This is due to a mistake of programmers who implemented early version of HTTP software.
Unfortunately, this header has become quite common and supported by many HTTP agents. It was not needed to duplicate the functionality of the Connection header but somehow a few programmers misunderstood the specification.
You can read more on this topic at this link: The Proxy-Connection: header is a mistake in how some