Fixed RequestUrl, it now adds API Key to the URL

[Albert-Gao]

Without this fix, it throws such error when invoking:

[GoogleGenerativeAI Error]: Error fetching from https://generativelanguage.googleapis.com/v1beta/models/gemini-1.5-flash:generateContent: [403 ] Method doesn't allow unregistered callers (callers without established identity). Please use API Key or other form of API consumer identity to call this API.

After applying the fix, it works now.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.