fix #399: Raise error for requests bigger than BaseRequest.MEMFILE_MAX

Some content-types are decoded in memory. To prevent memory exhaustion,
BaseRequest.MEMFILE_MAX was introduced. Prior to this patch, the request
body was silently truncated. This could cause some decoders to fail or
return wrong data. Bottle now raises a HTTPError(413, 'Request to large').
BaseRequest.MAX_PARAMS had the same problems.