SWF endpoints raise a 400 Bad Request for non-string types, and boto
doesn't enforce it as of today, so better have some safety nets in moto
to avoid this common mistake.
Example exception raised by Boto:
SWFResponseError: SWFResponseError: 400 Bad Request
{u'Message': u'class java.lang.Short can not be converted to an String',
u'__type': u'com.amazon.coral.service#SerializationException'}
