Fix up cfg attributes to work on an xor basis.
Previously, the cfg(any()) attributes would cause issues when
both native-tls and tls features were enabled. Now, https functions
and enum variants will only be created when tls xor native-tls are
enabled. Additionally, a compile error has been added for when
both tls and native-tls features are enabled.
This builds on 753d61b. Before we send a request, we can do a 1-byte
nonblocking peek on the connection. If the server has closed the
connection, this will give us an EOF, and we can take the connection out
of the pool before sending any request on it. This will reduce the
likelihood that we send a non-retryable POST on an already-closed
connection.
The server could still potentially close the connection between when we
make this check and when we finish sending the request, but this should
handle the majority of cases.
This is not a perfect solution. It works as long as we are not sending
any body bytes. We discover the error first when attempting to read
the response status line. That means we discover the error after
sending body bytes. To be able to re-send the body, we would need to
introduce a buffer to be able to replay the body on the next
request. We don't currently do that.
