Adding the fluent tag and time information as http headers

Hi,

I stumbled upon your lib and really like the concept. However, it appears some information is missing when sending the http request. The report is posted as json but the tag and date/time of the entry are missing.

After a quick look at the source code, it seems that indeed, that info is simply discarded when forging the request. In my case, I would find it interesting to have that info. Thinking about the best way to handle this, I thought about adding it via the http headers. Does this seems acceptable to you ?

```
X-Fluent-date: ENTRYDATETIME
X-Fluent-tag: my.fluent.tag3
```