SECURITY: Work around CURL insanity breaking POST parameters that start with '@'

SECURITY: Work around CURL insanity breaking POST parameters that start with '@'

CURL has a "feature" where passing array( 'foo' => '@bar' )
in CURLOPT_POSTFIELDS results in the contents of the file named "bar"
being POSTed. This makes it impossible to POST the literal string "@bar",
because array( 'foo' => '%40bar' ) gets double-encoded to foo=%2540bar.

Disable this "feature" by setting CURLOPT_SAFE_UPLOAD to true,
if available. According to the PHP manual, this option became
available in 5.5 and started defaulting to true in 5.6.
However, we support versions as low as 5.3, and this option
doesn't exist at all in 5.6.99-hhvm, which we run in production.

For versions where this option is not available (pre-5.5 versions
and HHVM), serialize POSTFIELDS arrays to strings. This works
around the issue because the '@' "feature" only works
for arrays, not strings, as of PHP 5.2. (We don't support pre-5.2
versions, and I've verified 5.6.99-hhvm behaves this way as well.)

Bug: T118032
Signed-off-by: Chad Horohoe <chadh@wikimedia.org>
Change-Id: I3f996e2eb87c7bd3b94ca9d3cc14a3e12f34f241