Issue captured in guzzlehttp version 6 supported by this library

This library supports guzzlehttp version ~6 in which
Deprecated: http_build_query(): Passing null to parameter #2 ($numeric_prefix) of type string is deprecated in vendor/guzzlehttp/guzzle/src/Client.php on line 445.

This issue has been fixed in guzzelhttp version ^7

@rajathans Please take a look at this issue and update the composer.json file.

Thanks!
