Added new Client::Get variant that combines a ContentReceiver with a new ResponseHandler

While trying to implement streaming of internet radio, where a ContentReceiver is needed to handle the audio data, I had the problem, that important information about the stream data is part of the HTTP header (e.g. size of audio chunks between meta data), so I added a ResponseHandler and a new Get variant, to gain access to the header before handling the first chunk of data.

The ResponseHandler can abort the request by returning false, in the same way as the ContentReceiver.

A test case was also added.

test/test.cc

@@ -242,6 +242,38 @@ TEST(ChunkedEncodingTest, WithContentReceiver) {
   EXPECT_EQ(out, body);
 }
 
+TEST(ChunkedEncodingTest, WithResponseHandlerAndContentReceiver) {
+  auto host = "www.httpwatch.com";
+  auto sec = 2;
+
+#ifdef CPPHTTPLIB_OPENSSL_SUPPORT
+  auto port = 443;
+  httplib::SSLClient cli(host, port, sec);
+#else
+  auto port = 80;
+  httplib::Client cli(host, port, sec);
+#endif
+
+  std::string body;
+  auto res =
+      cli.Get("/httpgallery/chunked/chunkedimage.aspx?0.4153841143030137", Headers(),
+              [&](const Response& response) {
+                EXPECT_EQ(200, response.status);
+                return true;
+              },
+              [&](const char *data, size_t data_length, uint64_t, uint64_t) {
+                body.append(data, data_length);
+                return true;
+              });
+  ASSERT_TRUE(res != nullptr);
+
+  std::string out;
+  httplib::detail::read_file("./image.jpg", out);
+
+  EXPECT_EQ(200, res->status);
+  EXPECT_EQ(out, body);
+}
+
 TEST(RangeTest, FromHTTPBin) {
   auto host = "httpbin.org";
   auto sec = 5;