fix(ext/fetch): better error if no content-type

The streaming WASM support code inspects the Response object's
Content-Type header but if that was missing, it failed with a fairly
inscrutable "String.prototype.toLowerCase called on null or undefined"
exception. Now it raises a more legible "Invalid WebAssembly content
type" exception.

cli/tests/unit/wasm_test.ts

@@ -48,6 +48,19 @@ unitTest(
   },
 );
 
+unitTest(
+  async function wasmInstantiateStreamingNoContentType() {
+    await assertThrowsAsync(
+      async () => {
+        const response = Promise.resolve(new Response(simpleWasm));
+        await WebAssembly.instantiateStreaming(response);
+      },
+      TypeError,
+      "Invalid WebAssembly content type.",
+    );
+  },
+);
+
 unitTest(async function wasmInstantiateStreaming() {
   let isomorphic = "";
   for (const byte of simpleWasm) {

ext/fetch/26_fetch.js

@@ -503,9 +503,10 @@
         // The spec is ambiguous here, see
         // https://github.com/WebAssembly/spec/issues/1138. The WPT tests
         // expect the raw value of the Content-Type attribute lowercased.
+        const contentType = res.headers.get("Content-Type");
         if (
-          StringPrototypeToLowerCase(res.headers.get("Content-Type")) !==
+          typeof contentType !== "string" ||
-            "application/wasm"
+          StringPrototypeToLowerCase(contentType) !== "application/wasm"
         ) {
           throw new TypeError("Invalid WebAssembly content type.");
         }