Fix write_int and write_short type validation

It will still die, just as before, but it now includes a *helpful* error message

Author: mdaniel

kafka/util.py

@@ -7,13 +7,19 @@
 
 
 def write_int_string(s):
+    if s is not None and not isinstance(s, str):
+        raise TypeError('Expected "%s" to be str\n'
+                        'data=%s' % (type(s), repr(s)))
     if s is None:
         return struct.pack('>i', -1)
     else:
         return struct.pack('>i%ds' % len(s), len(s), s)
 
 
 def write_short_string(s):
+    if s is not None and not isinstance(s, str):
+        raise TypeError('Expected "%s" to be str\n'
+                        'data=%s' % (type(s), repr(s)))
     if s is None:
         return struct.pack('>h', -1)
     elif len(s) > 32767 and sys.version < (2, 7):