Accept a Unix socket path with "//" in it

Client took any string with "//" in it that it could not use as a URL
for a bad URL, so a Unix socket path such as /run/app//api.sock gave an
invalid client. Only a string that starts with a scheme and "://", or
with "//", is taken for a URL now.

httplib.h

@@ -17988,8 +17988,10 @@ inline Client::Client(const std::string &scheme_host_port,
       cli_ = detail::make_unique<ClientImpl>(host, port, client_cert_path,
                                              client_key_path);
     }
-  } else if (scheme_host_port.find("//") != std::string::npos) {
+  } else if (scheme_host_port.compare(0, 2, "//") == 0 ||
-    // A URL, but not one this client can use, such as one with userinfo
+             scheme_host_port.find("://") < scheme_host_port.find('/')) {
+    // A URL, but not one this client can use, such as one with userinfo. A
+    // Unix socket path may have "//" further on.
     cli_ = detail::make_unique<detail::InvalidClientImpl>();
   } else {
     // NOTE: Update TEST(UniversalClientImplTest, Ipv6LiteralAddress)

test/test.cc

@@ -173,6 +173,8 @@ TEST_F(UnixSocketTest, pathname) {
   ASSERT_TRUE(svr.is_running());
 
   client_GET(pathname_);
+  // ".//x.sock", which is not taken for a URL
+  client_GET("./" + pathname_.substr(1));
 }
 
 #if defined(__linux__) ||                                                      \
@@ -15633,6 +15635,8 @@ TEST(HostAndPortPropertiesTest, UserinfoIsNotSupported) {
   httplib::Client cli("http://user:pass@localhost:1234");
   ASSERT_FALSE(cli.is_valid());
   EXPECT_FALSE(cli.Get("/"));
+
+  EXPECT_FALSE(httplib::Client("//user@localhost:1234").is_valid());
 }
 
 TEST(HostAndPortPropertiesTest, TrailingCharactersInPort) {