Problem description: If an API parameter has only 1 possible valid value, and it is passed with an incorrect value, the error message refers to its possible values in plural, even though only 1 possible option is mentioned in it. For example, the error message obtained when specifying an incorrect value of "output" parameter of autoregistration.get API: "Invalid parameter \"/output\": value must be one of extend."
Furthermore, other get APIs don't have any validation for this parameter at all.

Steps to reproduce:
Send the following API:

{
   "jsonrpc": "2.0",
    "method": "autoregistration.get",
    "params": {
        "output": "incorrect_value"
    },
    "auth": "e5fef1c5302e22e4d4b56bafd621ac93",
    "id": 1
}

Result:

"error": {
        "code": -32602,
        "message": "Invalid params.",
        "data": "Invalid parameter \"/output\": value must be one of extend."

Expected:
"data": "Invalid parameter \"/output\": value must be equal to 'extend'."

Send any other get API with incorrect value of "output" parameter:

{
    "jsonrpc": "2.0",
    "method": "host.get",
    "params": {
        "output": "please_do_not_extend"
    },
    "auth": "e5fef1c5302e22e4d4b56bafd621ac93",
    "id": 1
}

Result: a list of the referred entities ids is returned:

{
    "jsonrpc": "2.0",
    "result": [
        {
            "hostid": "10084"
        },
        {
            "hostid": "10264"
        }
    ],
    "id": 1
}

Expected: an error stating that the passed 'extend' parameter value is incorrect should be returned, like in the case of autoregistration.get API.