fix bug when using both feature_mask and initial_regressor

This fixes the second bug discussed in pull request #191, which arose when the feature mask weights were overwriting and initial regressor weights loaded.  In the case where just the feature mask is specified, the weights from that model are used as the initial weights.

Also fixes a bug where if `save_load()` was called twice, such as when using an initial model and separate feature mask, it was double allocating and leaking a copy of the weight vector, in the `initialize_regressor` function in `parse_regressor.cc`.

Adds a test for the initial regressor + feature mask case.