Fix infinite loop when same object path is generated. fix #48

S3 plugin generates unique path for avoiding path conflict.
But if users set s3_object_key_format without index,
S3 plugin tries to upload events with same path infinitely.
To fix this issue, S3 plugin raises an exception when detects same path.
If users don't use %{index}, users should set <secondary> for preventing log lost.
Merge pull request #50 from fluent/fix-infinite-loop-without-index

Fix infinite loop when same object path is generated. fix #48