parse_spreadsheet_contents issue

[incubo4u]

https://github.com/quip/quip-api/blame/19f3b32a05ed092a70dc2c616e214aaff8a06de2/python/quip.py#L712

I believe this approach might not return the correct value for a cell with multiline content.

[incubo4u]

#104

this is a potential fix