Fixed #1131 using old endpoint naming

[ezimuel]

This PR fixes #1131 using the old endpoint naming convention used in elasticsearch-php <= 6.7. In particular, it adds a check in the endpoint generator script (util/GenerateEndpoints.php) to use this naming conversion:

const BC_CLASS_NAME = [
    'Cat\Nodeattrs'      => 'NodeAttrs',
    'Indices\Forcemerge' => 'ForceMerge',
    'Mtermvectors'       => 'MTermVectors',
    'Termvectors'        => 'TermVectors'
];

[kbond]

@ezimuel this solves #1131 for me!

[ezimuel]

Thanks @kbond