Add tests for handling bad content encoding in fetch

fetch/content-encoding/bad-gzip-body.any.js

@@ -0,0 +1,20 @@
+promise_test((test) => {
+    return fetch("resources/bad-gzip-body.py").then(res => {
+      assert_equals(res.status, 200);
+    });
+}, "Fetching a resource with bad gzip content should still resolve");
+
+[
+  "arrayBuffer",
+  "blob",
+  "formData",
+  "json",
+  "text"
+].forEach(method => {
+  promise_test(t => {
+    return fetch("resources/bad-gzip-body.py").then(res => {
+      assert_equals(res.status, 200);
+      return promise_rejects(t, new TypeError(), res[method]());
+    });
+  }, "Consuming the body of a resource with bad gzip content with " + method + "() should reject");
+});

fetch/content-encoding/resources/bad-gzip-body.py

@@ -0,0 +1,3 @@
+def main(request, response):
+    headers = [("Content-Encoding", "gzip")]
+    return headers, "not actually gzip"