reaction stoichiometry doubled on deletion

[aebrahim]

The following sample code demonstrates the bug

import cobra.test
model = cobra.test.create_test_model()
pgi = model.reactions.get_by_id("PGI")
print "before deletion:", pgi.reaction
model.remove_reactions([pgi])
print "after deletion:", pgi.reaction

outputs:

before deletion: 1.0 g6p_c <=> 1.0 f6p_c
after deletion: 2.0 g6p_c <=> 2.0 f6p_c