Wrong httpx `Content-Length` header breaks `get_account_info`

[pde-rent]

Hi, thanks for your work on the client.
I noticed strange behaviors while reading account data from arbitrary contracts, it seems that the solana official RPC is unhappy with httpx's injected content-length header.

client = AsyncClient("https://api.mainnet-beta.solana.com")
account_info = await client.get_account_info(Pubkey.from_string("9tXiuRRw7kbejLhZXtxDxYs2REe43uH2e7k1kocgdM9B"))

The above code will generate this request on httpx.AsyncClient's side:

{
  "url": "https://api.mainnet-beta.solana.com",
  "headers": {
    "Content-Type": "application/json",
    "Host": "api.mainnet-beta.solana.com",
    "Accept": "*/*",
    "Accept-Encoding": "gzip, deflate",
    "Connection": "keep-alive",
    "User-Agent": "python-httpx/0.28.1",
    "Content-Length": "194"
  },
  "content": {
    "method": "getAccountInfo",
    "jsonrpc": "2.0",
    "id": 0,
    "params": [
      "9tXiuRRw7kbejLhZXtxDxYs2REe43uH2e7k1kocgdM9B",
      {
        "encoding": "base64",
        "dataSlice": null,
        "commitment": "finalized",
        "minContextSlot": null
      }
    ]
  }
}

Here is a curl equivalent for quick testing:

curl -X POST https://api.mainnet-beta.solana.com \
-H "Host: api.mainnet-beta.solana.com" \
-H "Accept: */*" \
-H "Accept-Encoding: gzip, deflate" \
-H "Connection: keep-alive" \
-H "User-Agent: python-httpx/0.28.1" \
-H "Content-Type: application/json" \
-H "Content-Length: 194" \
-d '{
  "method": "getAccountInfo",
  "jsonrpc": "2.0",
  "id": 0,
  "params": [
    "9tXiuRRw7kbejLhZXtxDxYs2REe43uH2e7k1kocgdM9B",
    {
      "encoding": "base64",
      "dataSlice": null,
      "commitment": "finalized",
      "minContextSlot": null
    }
  ]
}'

This breaks, Solana's RPC returning curl: (92) HTTP/2 stream 0 was not closed cleanly: PROTOCOL_ERROR (err 1)

If the Content-Length header is removed or fixed (set to 261 in this example), the query is processed:

{"jsonrpc":"2.0","result":{"context":{"apiVersion":"2.0.21","slot":315171374},"value":{"data":["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","base64"],"executable":false,"lamports":5443300,"owner":"whirLbMiicVdio4qvUfM5KAg6Ct8VwpYzGff3uctyCc","rentEpoch":18446744073709551615,"space":653}},"id":0}

Not sure if the header value not matching the actual content size is due to solana-py or httpx