Disable cURL "Expect:" header

Summary:
Fixes T2612. That task has a pretty good explanation of this issue. It's been on my radar for a while, but basically:

• By default, cURL sends "Expect: 100 continue" with some requests (I believe POSTs over a certain size).
• By default, lighttpd rejects "Expect: 100 continue" with a 417 error and kills the request.
• There's no practical benefit for us to sending this header (it's "nicer", but probably a little slower if anything).
• The "correct" way to disable this behavior is to add an empty "Expect" header.

Test Plan: Confirmed resolved by @jucrouzet in T2612.

Reviewers: vrana, chad

Reviewed By: chad

CC: jucrouzet, aran

Maniphest Tasks: T2612

Differential Revision: https://secure.phabricator.com/D5136