Reuse persistent connection

When using a persistent connection, the point is to reuse the underlying
socket. Prior to this commit, the socket would be thrown away before
each request unless `net_http_connect_on_start` is `true`.

This commit will preserve the previously started socket if the socket
represents an actual connection.

lib/webmock/http_lib_adapters/net_http.rb

@@ -98,12 +98,8 @@ def request(request, body = nil, &block)
               after_request.call(response)
             }
             if started?
-              if WebMock::Config.instance.net_http_connect_on_start
-                super_with_after_request.call
-              else
-                start_with_connect_without_finish
-                super_with_after_request.call
-              end
+              ensure_actual_connection
+              super_with_after_request.call
             else
               start_with_connect {
                 super_with_after_request.call
@@ -131,12 +127,8 @@ def start_without_connect
         end
 
 
-        def start_with_connect_without_finish
-          if @socket
-            @socket.close
-            @socket = nil
-          end
-          do_start
+        def ensure_actual_connection
+          do_start if @socket.is_a?(StubSocket)
         end
 
         alias_method :start_with_connect, :start

spec/acceptance/net_http/net_http_shared.rb

@@ -68,8 +68,11 @@
 
       if WebMock::Config.instance.net_http_connect_on_start
         expect(sockets.length).to eq(1)
+        expect(sockets.to_a[0]).to be_a(Net::BufferedIO)
       else
-        expect(sockets.length).to eq(4)
+        expect(sockets.length).to eq(2)
+        expect(sockets.to_a[0]).to be_a(StubSocket)
+        expect(sockets.to_a[1]).to be_a(Net::BufferedIO)
       end
 
       expect(sockets.all?(&:closed?)).to be(true)