Consistently use CreateProxyUri in WebProxy ctors

[MihaZupan]

WebProxy has new(Uri Address), new(string Address), new(string Host, int Port) constructors that all behave differently.

The new(string) overload will check if the address is an absolute Uri and add a "http://" prefix otherwise.
The new(string, int) overload will blindly concat the host and port $"http://{Host}:{Port}".

If you happen to create a WebProxy object like new WebProxy("socks5://127.0.0.1", 9050), what you actually end up with is a proxy to "http://socks5://127.0.0.1:9050" - an http proxy with the host socks5 and a default port.

We should either check if the host is already an absolute Uri like we do for other overloads (this PR), or we should throw. socks5://127.0.0.1 is not a valid host, it just happens to be interpreted as:
Host: socks5
Port: : (empty port is allowed and means "default for the scheme")
Path: //127.0.0.1

cc: @ManickaP who noticed our blog posts announcing socks support have broken code snippets.

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Issue Details

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WebProxy has new(Uri Address), new(string Address), new(string Host, int Port) constructors that all behave differently.

The new(string) overload will check if the address is an absolute Uri and add a "http://" prefix otherwise.
The new(string, int) overload will blindly concat the host and port $"http://{Host}:{Port}".

If you happen to create a WebProxy object like new WebProxy("socks5://127.0.0.1", 9050), what you actually end up with is a proxy to "http://socks5://127.0.0.1:9050" - an http proxy with the host socks5 and a default port.

We should either check if the host is already an absolute Uri like we do for other overloads (this PR), or we should throw. socks5://127.0.0.1 is not a valid host, it just happens to be interpreted as:
Host: socks5
Port: : (empty port is allowed and means "default for the scheme"
Path: //127.0.0.1

cc: @ManickaP who noticed our blog posts announcing socks support have broken code snippets.

Author:	MihaZupan
Assignees:	-
Labels:	area-System.Net
Milestone:	7.0.0

[ManickaP]

That was super fast 😄