The isDisconnected() method must return a boolean.

In some rare cases, $this->_disconnect can be set to null instead of a
boolean (it appears when the connection is shutdown unproperly). So we
force to compute a boolean.
[auto] Merge from Socket

    • #742fe31 — The isDisconnected() method must return a boolean. (Ivan Enderlin, 2013-09-09)

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Merge remote branch 'library-socket/master'