Better check for json format parameters

[Dimitar Misev]

In encode/decode there are two different formats for the format parameters (third parameter of these functions): key/value (deprecated) and json.

Currently, we try to parse json; if this fails, we print a warning and try to parse key/value then.

This is not the best approach, as sometimes the format parameters might be json, but fail to parse because the json is invalid. In this case rasdaman should throw an exception instead of printing a warning.

A better test for whether the format parameters are json, is to check if the first non-whitespace character is '{'.