[CLIENT] Correctly return Hashes from `perfom_request` with the Curb transport

Because of the change introduced in elasticsearch/elasticsearch-ruby#80c48dd,
responses from the Curb transport were not correctly deserialized into Hash from JSON string.

That's because we actually haven't returned any response headers in the `Curb#perform_request` method.

Since Curb doesn't return any response headers as a Hash (see related), we simply check for:

    header_str =~ /\/json/

in `Curb#perform_request`.

This seems to have noticeable impact on the raw performance, adding ~ 10ms to _mean_ times
when running the client benchmark test. On the other hand, previously we did check body
whether it "looks like a JSON", which definitely has been influencing the end times,
so hopefully it balances out.

Once Curb adds response headers as a Hash, the code should change appropriately.

Closes: #36
Related: taf2/curb#184