Flush interval doesn't flush buffer if it is not full

[Dim0N22]

The doc for _flushInterval says:

// Maximum number of seconds points can be held in buffer before are written to the db.
// Buffer is flushed when it reaches batch size or when flush interval runs out.
uint16_t _flushInterval;

However, it seems that due to passing true to InfluxDBClient::flushBufferInternal(bool flashOnlyFull), flushBufferInternal will actually do nothing even when it's time to flush.

I think we need to change this line

InfluxDB-Client-for-Arduino/src/InfluxDbClient.cpp

Line 366 in eaee8e3

return flushBufferInternal(true);

to

return flushBufferInternal(false);

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I also would be happy to cut a PR, but I don't know how to setup the environment for testing and run unit tests and unfortunately README doesn't help a lot (I use PlatformIO and consume the library from my project).

[vlastahajek]

@Dim0N22, thanks for raising issues regarding flash timeout.
The suggested fix above cannot be done this way because when flushing all batches after connection error occurs, flushed are only full buffers.