Revision c559f8ad7060d7ea22e13b7fb5c301225f6b3b19

Fix Error if only one Private Data

When adding only one private data, result returned by Payline webservice won't contain `privateData`
as an array of data, but the data itself.

With only one private data, ['privateDataList']['privateData'] is not an
array but the data itself.

Several private data:
[
['key' => 'foo', 'value' => 'bar'],
['key' => 'baz', 'value' => '123456'],
]

One private data:
[
'key' => 'foo',
'value' => 'bar',
]

Fix #1