Refine the streaming, add UT, add error handling.

unittest/response_builder_unittest.cc

@@ -0,0 +1,17 @@
+#include "gtest/gtest.h"
+
+#include "webcc/response_builder.h"
+
+// Empty body should also have `Content-Length` header.
+TEST(ResponseBuilderTest, EmptyBody) {
+  using namespace webcc;
+
+  auto response = ResponseBuilder{}.OK()();
+
+  bool existed = false;
+  const auto& value = response->GetHeader(headers::kContentLength, &existed);
+
+  EXPECT_TRUE(existed);
+
+  EXPECT_EQ("0", value);
+}