Fix Maniphest Excel export for tasks with descriptions beginning with "="
Summary: Fixes T2369. If you create a task with a description like = Header =, the Excel writer interprets it as a formula. Explicitly set the cell types to strings to avoid this.
Test Plan: Exported a task with the description =1,; no exception after this patch.
Reviewers: btrahan, chad, vrana
Reviewed By: chad
CC: aran
Maniphest Tasks: T2369
Differential Revision: https://secure.phabricator.com/D4567