fix bug in mat->sbml translation

Reading in a mat model did not set the compartment, and writing an SBML
model would fail if compartment was None, causing the following code to
error out:

```
>>> import cobra.test
>>> cobra.io.write_sbml_model(cobra.io.load_matlab_model(cobra.test.ecoli_mat,
    "test.xml")
```

This commit fixes both issues and adds a unit test to ensure all model
loaders appropriately set the compartment.