Fix SSE headers symbol error when Headers object is passed

Handle Headers object properly in eventSourceInit fetch function by converting
it to a plain object before spreading. This fixes the ByteString conversion
error that occurs when Headers contains symbols.

Closes #150 and #148

Co-authored-by: Amp <[email protected]>
Amp-Thread-ID: https://ampcode.com/threads/T-424be391-89c5-403e-946a-f5463bb562d5

Author: geelen

src/lib/utils.ts

@@ -279,7 +279,7 @@ export async function connectToRemoteServer(
         fetch(url, {
           ...init,
           headers: {
-            ...(init?.headers as Record<string, string> | undefined),
+            ...(init?.headers instanceof Headers ? Object.fromEntries(init.headers.entries()) : init?.headers),
             ...headers,
             ...(tokens?.access_token ? { Authorization: `Bearer ${tokens.access_token}` } : {}),
             Accept: 'text/event-stream',