fix(fetch): capitalize HTTP method names; (#6395)

lib/adapters/fetch.js

@@ -122,7 +122,7 @@ export default isFetchSupported && (async (config) => {
       let requestContentLength = await resolveBodyLength(headers, data);
 
       let _request = new Request(url, {
-        method,
+        method: 'POST',
         body: data,
         duplex: "half"
       });
@@ -146,7 +146,7 @@ export default isFetchSupported && (async (config) => {
     request = new Request(url, {
       ...fetchOptions,
       signal: composedSignal,
-      method,
+      method: method.toUpperCase(),
       headers: headers.normalize().toJSON(),
       body: data,
       duplex: "half",