Validate settings.

[Lukasa]

H2Spec correctly flags that we don't really do any validation of settings values:

    6.5.2. Defined SETTINGS Parameters
      × SETTINGS_ENABLE_PUSH (0x2): Sends the value other than 0 or 1
        - The endpoint MUST respond with a connection error of type PROTOCOL_ERROR.
          Expected: GOAWAY frame (ErrorCode: PROTOCOL_ERROR)
                    Connection close
            Actual: SETTINGS frame (Length: 0, Flags: 1)
      × SETTINGS_INITIAL_WINDOW_SIZE (0x4): Sends the value above the maximum flow control window size
        - The endpoint MUST respond with a connection error of type FLOW_CONTROL_ERROR.
          Expected: GOAWAY frame (ErrorCode: FLOW_CONTROL_ERROR)
                    Connection close
            Actual: SETTINGS frame (Length: 0, Flags: 1)
      × SETTINGS_MAX_FRAME_SIZE (0x5): Sends the value below the initial value
        - The endpoint MUST respond with a connection error of type PROTOCOL_ERROR.
          Expected: GOAWAY frame (ErrorCode: PROTOCOL_ERROR)
                    Connection close
            Actual: SETTINGS frame (Length: 0, Flags: 1)
      × SETTINGS_MAX_FRAME_SIZE (0x5): Sends the value above the maximum allowed frame size
        - The endpoint MUST respond with a connection error of type PROTOCOL_ERROR.
          Expected: GOAWAY frame (ErrorCode: PROTOCOL_ERROR)
                    Connection close
            Actual: SETTINGS frame (Length: 0, Flags: 1)