switch to '\n' for EventSourceResponse separator

[khimaros]

this fixes compatibility with some OpenAI clients, including BetterChatGPT (ztjhz/BetterChatGPT#537). i'm not sure if this will cause problems with other clients. i'm also working with the client to see if they are willing to handle these separators more gracefully.

[khimaros]

note that BetterChatGPT client still has trouble with sse-starlet ping messages as described in ztjhz/BetterChatGPT#538 -- the hacky workaround i'm using locally for this is to set the ping=100000 on the EventSourceResponse -- sse-starlet doesn't seem to provide a way to disable these entirely so it seems like a client side solution would be ideal.

[abetlen]

@khimaros makes sense, I'm assuming the \n seperator is what's used by the openai api when doing server sent events / streaming.

[abetlen]

Thank you, I'll go ahead and merge this.