fix handling of request bodies

client/tests/scripts/test_post.js

@@ -0,0 +1,10 @@
+async function test() {
+  let payload = "hello".repeat(100);
+  let r = await libcurl.fetch("https://httpbin.org/anything", {
+    method: "POST",
+    body: new Blob([payload], {type: "text/plain"})
+  });
+  assert(r.status === 200, "wrong status");
+  let json = await r.json();
+  assert(json.data === payload, "server reports wrong payload");
+}