update contract return type verification

[ixje]

What current issue(s) does this address, or what feature is it adding?
while #978 fixed Contract return type validation for the block at hand, it broke a historical validation. When using Python enums a ValueError is thrown when trying to create an enum from an invalid value
e.g.

class MyEnum(Enum):
   bla = 0x1

t1 = MyEnum(1) # OK
t2 = MyEnum(2) # raises ValueError

In C# it actually doesn't care as long as the value you're trying to cast fits in the datatype (in the case of ContractParameter a byte). In #978 the return_type value was 2 bytes and failed to cast to a 1 byte enum in C#. The solution used for that block was ok, but fails for block 846536. This PR resolves that.

How did you solve this problem?
change validation logic

How did you make sure your solution works?
audit now also passes for block 846536

Are there any special changes in the code that we should be aware of?

Please check the following, if applicable:

• Did you add any tests?
• Did you run make lint?
• Did you run make test?
• Are you making a PR to a feature branch or development rather than master?
• Did you add an entry to CHANGELOG.rst? (if not, please do)

[coveralls]

Coverage increased (+0.003%) to 85.281% when pulling 003ff11 on ixje:update-return-type-checking into 9bf3c1e on CityOfZion:development.