ensure atom scheme

[ruslandoga]

After elixir-ecto/ecto@03b0597 the scheme might arrive as a string from the passed in URL which would result in an error like

10:58:03.114 [error] GenServer #PID<0.584.0> terminating
** (FunctionClauseError) no function clause matching in Mint.Negotiate.transport_connect/4
    (mint 1.4.2) lib/mint/negotiate.ex:48: Mint.Negotiate.transport_connect("http", "127.0.0.1", 8123, [mode: :passive, transport_opts: [keepalive: true, show_econnreset: true]])
    (clickhousex 0.5.0) lib/clickhousex/protocol.ex:39: Clickhousex.Protocol.connect/1
    (db_connection 2.4.3) lib/db_connection/connection.ex:82: DBConnection.Connection.connect/2
    (connection 1.1.0) lib/connection.ex:622: Connection.enter_connect/5
    (stdlib 3.17.1) proc_lib.erl:226: :proc_lib.init_p_do_apply/3
Last message: nil
State: Clickhousex.Protocol

This PR ensures the scheme passed is an atom.