Add tests for ERC20 as per pdaian's suggestions

[spalladino]

On his audit for Tether, @pdaian identified some code paths in the ERC20 token contract which did not have explicit unit tests to exercise them.

Add a test case for each of the following:

• Testing the code path in approve that does not allow non-zero approval with an
  existing non-zero allowance. Furthermore, the documentation does not sufficiently
  warn contract implementers to use the "set to 0, check, then reset" pattern for more
  than one approval operation.
• transferFrom should also check for the ability to transfer more than balance when
  allowance is correctly set.
• Tests with zero values are generally missing, though manual inspection shows that
  the class’s guarantees will not be violated with such values. Interestingly, the transferFrom
  and transfer methods impose a payload size restriction representing such
  a check, for which a test is missing. The allowance method does not feature this
  modifier, allowing for the creation of transfer approvals that can never be executed.
  This does not seem to be a substantial security risk in the contract.
• Tests with zero-length (null) addresses are generally missing, though manual inspection
  shows that the class’s guarantees will not be violated with such addresses.

[facuspagnuolo]

Fixed in #686