serializer: Fail if dictionary uses invalid keys

JSON only supports a very restricted set of types for dictionary keys,
among them strings, booleans and “null”. Integers and floats are
converted to strings. Since this can cause a lot of confusion in Python,
this check raises an exception if a caller tries to use such types.

Since the pre-Python 2.6 “simplejson” module doesn't support overriding
the function where the conversion takes place this check can only be
done for the newer “json” module.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>