Disable case-changing behavior in Eventlet

RFC 2616 says that HTTP header fields are case-insensitive.  However, there are
some S3 clients who don't accept normalized header by Swift and Eventlet.  For
example, AWS Java SDK expects that an etag header is 'ETag', not 'Etag'.

This patch disables Eventlet's header capitalization so that the swift3
middleware can normalize the response headers as those clients expect.

Note that this change requires a fix for Eventlet, which will be included in
the next Eventlet release (v0.15).

Change-Id: I6d3428b0dafef776bdb3ebac7639b3126fa5e60d
Merge "Disable case-changing behavior in Eventlet"