Fix MiniBatchDictLearning partial_fit

This PR overides and completes #2600 : it adds to the syntax fix of @yarikoptic a logical fix to the partial_fit of MiniBatchDictLearning, so that now the online code actually is equivalent to the batch code, and the tests pass.

This should make MiniBatchDictLearning, when used in online setting, much more performant in the sens of estimation.