Revert breaking change in HttpClient

This reverts commit 85c2994e70428f4957858d0c934496e40f3bbc09.

This change breaks issue creation. An array of labels in the parameters to `api('issue')->create()`

    'labels'=>array('label1','label2)

should be encoded as:

    "labels":{"label1","label2"}

but with `JSON_FORCE_OBJECT` it is encoded as

    "labels":{"0":"label1","1":"label2"}

which fails validation and results in 422 Unprocessable Entity.