log options timestamp is ignored

The `timestamp` passed to the `log` method via options here
https://github.com/logdna/ruby/blob/v1.5.0/lib/logdna.rb#L79-L81

is ignored here
https://github.com/logdna/ruby/blob/v1.5.0/lib/logdna/client.rb#L64

Seems that this line should be changed to something like `timestamp: opts[:timestamp]`

Start in lib/logdna.rb around lines 79-81 to trace the timestamp option into lib/logdna/client.rb around line 64. Confirm that the supplied timestamp is used when building the log request, and verify that the request preserves the option value instead of ignoring it.

Written by the indexing model from the issue text.