Fix #977: Handle the exception as structured even if the reason is null

[oogFranz]

Close #977
The case of the issue #977 is handling structured exception as semi-structured exception which only occurs before Elasticsearch 2.0.

Ordinal code checks whether $error['error']['reason'] is set.

elasticsearch-php/src/Elasticsearch/Connections/Connection.php

Line 683 in c65c1fb

if (isset($error['error']['reason']) === true) {

Unfortunately, master_not_discovered_exception is thrown with blank error reason.
https://github.com/elastic/elasticsearch/blob/16bfdcacc0172264db9b32aa6edfa8b5c327f3b4/server/src/main/java/org/elasticsearch/discovery/MasterNotDiscoveredException.java#L31

To avoid the problem, use array_key_exists() alter isset().
To make the logic simple, I refactored the if-statements.
Check whether the exception is structured or not first.
Then distinguish whether the exception is structured exception after Elasticsearch 2.0.

[ezimuel]

@oogFranz I added a unit test to cover the #977 issue and I applied an extra json_encode() to be more defensive for future changes. Thanks for your contribution!