Preserve milli-precision in non-prepared datetimes

When a datetime was used for a 'timestamp' value in a non-prepared
statement, all sub-second precision was lost.  After this change,
millisecond level precision will be kept.

cassandra/decoder.py

@@ -1,4 +1,5 @@
 from binascii import hexlify
+import calendar
 from collections import namedtuple
 import datetime
 import logging
@@ -788,7 +789,8 @@ def cql_encode_object(val):
 
 
 def cql_encode_datetime(val):
-    return "'%s'" % val.strftime('%Y-%m-%d %H:%M:%S-0000')
+    timestamp = calendar.timegm(val.utctimetuple())
+    return str(long(timestamp * 1e3 + getattr(val, 'microsecond', 0) / 1e3))
 
 
 def cql_encode_date(val):

tests/integration/test_types.py

@@ -131,7 +131,7 @@ class TypeTests(unittest.TestCase):
 
         v1_uuid = uuid1()
         v4_uuid = uuid4()
-        mydatetime = datetime(2013, 1, 1, 1, 1, 1)
+        mydatetime = datetime(2013, 12, 31, 23, 59, 59, 999000)
 
         params = [
             "sometext",