Don't upload timestamps if SDK client didn't specify timestamps

When uploading a file, the SDK should only pass "content-created-at" and
"content-modified-at" timestamps if they were explicitly specified by
the user.  This allow Box to have intelligent default behavior for
timestamps.
Fix to previous commit

Some fixes to previous timestamp-related commit.
Merge pull request #102 from mmorearty/no-explicit-timestamps

Don't upload timestamps if SDK client didn't specify timestamps