Fix bug with HTTP entities with `Content-Length: 0`.

The entity-body would never be marked "done", breaking the pipeline for subsequent requests/responses.

(In practice `Content-Length: 0` is rare since normally only GET requests don't have content and they don't pass `Content-Length` at all.)

c++/src/kj/compat/http-test.c++

@@ -850,6 +850,47 @@ kj::ArrayPtr<const HttpTestCase> pipelineTestCases() {
       },
     },
 
+    // Throw a zero-size request/response into the pipeline to check for a bug that existed with
+    // them previously.
+    {
+      {
+        "POST /foo HTTP/1.1\r\n"
+        "Content-Length: 0\r\n"
+        "\r\n",
+
+        HttpMethod::POST, "/foo", {}, uint64_t(0), {},
+      },
+      {
+        "HTTP/1.1 200 OK\r\n"
+        "Content-Length: 0\r\n"
+        "\r\n",
+
+        200, "OK", {}, uint64_t(0), {}
+      },
+    },
+
+    // Also a zero-size chunked request/response.
+    {
+      {
+        "POST /foo HTTP/1.1\r\n"
+        "Transfer-Encoding: chunked\r\n"
+        "\r\n"
+        "0\r\n"
+        "\r\n",
+
+        HttpMethod::POST, "/foo", {}, nullptr, {},
+      },
+      {
+        "HTTP/1.1 200 OK\r\n"
+        "Transfer-Encoding: chunked\r\n"
+        "\r\n"
+        "0\r\n"
+        "\r\n",
+
+        200, "OK", {}, nullptr, {}
+      },
+    },
+
     {
       {
         "POST /bar HTTP/1.1\r\n"

c++/src/kj/compat/http.c++

@@ -1369,12 +1369,15 @@ public:
 class HttpFixedLengthEntityWriter final: public kj::AsyncOutputStream {
 public:
   HttpFixedLengthEntityWriter(HttpOutputStream& inner, uint64_t length)
-      : inner(inner), length(length) {}
+      : inner(inner), length(length) {
+    if (length == 0) inner.finishBody();
+  }
   ~HttpFixedLengthEntityWriter() noexcept(false) {
     if (length > 0) inner.abortBody();
   }
 
   Promise<void> write(const void* buffer, size_t size) override {
+    if (size == 0) return kj::READY_NOW;
     KJ_REQUIRE(size <= length, "overwrote Content-Length");
     length -= size;
 
@@ -1384,6 +1387,7 @@ public:
     uint64_t size = 0;
     for (auto& piece: pieces) size += piece.size();
 
+    if (size == 0) return kj::READY_NOW;
     KJ_REQUIRE(size <= length, "overwrote Content-Length");
     length -= size;
 
@@ -1391,6 +1395,7 @@ public:
   }
 
   Maybe<Promise<uint64_t>> tryPumpFrom(AsyncInputStream& input, uint64_t amount) override {
+    if (amount == 0) return Promise<uint64_t>(uint64_t(0));
     KJ_REQUIRE(amount <= length, "overwrote Content-Length");
     length -= amount;