Added Int, Short and Long Serializer Tests

#589

Will continue to add commits for other types in this pull request.

For number types, the test cases are {0, 1, -1, Type.MAX_VALUE, Type.MIN_VALUE, random_value, -random_value}. 

The Type.MIN_VALUE is negative.

random_value may be positive or negative depending on how it was generated. If random_value is negative , then -random_value is positive; and, vice versa. So both negative and positive cases are covered.

I left unused imports in the StringSerializerTest. Is it appropriate to use this pull request to fix it too?