SendButton should pass disabled attribute to IconButton

[Dpetters]

I am attempting to use the "sendBoxButtonColorOnFocus" styleset property, but it doesn't appear to get used because even if I set disabled=true on the ReactWebChat component, the SendButton is still enabled (whereas the TextBox becomes disabled). Thus the following styling does not apply:

    '&:disabled svg': {
      fill: sendBoxButtonColorOnDisabled
    }

I believe in SendButton.js, the disabled attribute should be set on the IconButton element in the way that it is done in MicrophoneButton and UploadButton.

[Dpetters]

Thanks guys for fixing this so quickly! You rock.