stream option should take any callable not just a proc

[obie]

Due to the way that Ruby closures work for procs, it's quite limiting to limit the stream interface to just Proc objects. This PR makes a tiny change so that callable objects work too. (Anything that implements a call method.)

Here's a simple example from my project MagmaChat:

  class StreamProcessor
    def initialize(bot:, buffer:, message:)
      Rails.logger.info "💦💦💦 initializing stream processor for message: #{message} 💦💦💦"
      @buffer = buffer
      @message = message
    end

    def call(chunk, _bytesize=nil)
      Rails.logger.info "💦💦💦 streaming chunk: #{chunk} (#{_bytesize} bytes) 💦💦💦"
      if new_content = chunk.dig("choices", 0, "delta", "content")
        @buffer << new_content
        ActiveRecord::Base.logger.silence do
          @message.update(content: @message.content + new_content)
        end
      end
    end
  end

[alexrudall]

Nice. thanks @obie