should default to content_encoding = "gzip" for influxdb

Relevant telegraf.conf:

[[outputs.influxdb]]
  ## HTTP Content-Encoding for write request body, can be set to "gzip" to
  ## compress body or "identity" to apply no encoding.
  # content_encoding = "identity"

System info:

Telegraf 1.14

Expected behavior:

gzip compression by default

Actual behavior:

no data compression

Additional info:

Need to make sure this won't break any old InfluxDB servers; but as far as I can tell it's not going to be an issue for any version of InfluxDB.