Fix case where string encoding overhead is larger than needed

According to the spec "serializers SHOULD use the format which
represents the data in the smallest number of bytes". This is not true
for strings between 32 and (2^8)-1 bytes which has an encoding
overhead of 3 bytes instead of the required 2.

My guess is that the somewhat strange conditional in fallback.py:598
(and cython equivalent) is to make sure that old raw type test
cases in test_case.py that test for encoding overhead succeeds.

This commit updates the test case to test for the encoding overhead
for both str and bin types of various lengths.