Streaming exception handling

[linhaojun857]

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Describe the bug When the data returned by SSE streaming starts with "data:" (not "data: "), streaming processing fails.

To Reproduce Steps to reproduce the behavior, including any relevant code snippets.

Expected behavior A clear and concise description of what you expected to happen.

Screenshots/Logs If applicable, add screenshots to help explain your problem. For non-graphical issues, please provide any relevant logs or stack traces.

Environment (please complete the following information):

• go-openai version: [e.g. v1.12.0]
• Go version: [e.g. 1.18]
• OpenAI API version: [e.g. v1]
• OS: [e.g. Ubuntu 20.04]

Additional context Add any other context about the problem here.