Fix SSL verification error

[bughandler]

This is a dedicated PR to fix #6900.

As I said in the issue, this looks like a bug of requests_toolbelt but turns out that it belongs to the requests itself.
Basically, what HostHeaderSSLAdapter does is quite simple:

    def send(self, request, **kwargs):
        # HTTP headers are case-insensitive (RFC 7230)
        host_header = None
        for header in request.headers:
            if header.lower() == "host":
                host_header = request.headers[header]
                break

        connection_pool_kwargs = self.poolmanager.connection_pool_kw

        if host_header:
            connection_pool_kwargs["assert_hostname"] = host_header
        elif "assert_hostname" in connection_pool_kwargs:
            # an assert_hostname from a previous request may have been left
            connection_pool_kwargs.pop("assert_hostname", None)

        return super(HostHeaderSSLAdapter, self).send(request, **kwargs)

As you can see, it simply set assert_hostname to the custom SNI for the urllib3.poolmanager.connection_pool_kw.

When the proxy is set, the function adapters.py!proxy_manager_for() forgets to copy the connection_pool_kwargs property from the current poolmanager, which will cause the SSL verification exception when the user is doing the request with the custom SNI and self-signed certificate.

PS: It's my first PR, sorry if my workflow doesn't meet your requirements.