logging: throw when a circular reference is used in an entry

[stephenplusplus]

Fixes #1354

Before

var entry = {
  val: true
};
entry.circularReference = entry;

var entry = log.entry(resource, circularReference);

log.write(entry, ...);

RangeError: Maximum call stack size exceeded
  ... Long stack trace ...

After

var entry = {
  val: true
};
entry.circularReference = entry;

var entry = log.entry(resource, circularReference);

log.write(entry, ...);

Error: The JSON data for this entry has a circular reference.

[coveralls]

Coverage remained the same at 100.0% when pulling 8ce0a2c on stephenplusplus:spp--1354 into 009bc99 on GoogleCloudPlatform:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 8f0f942 on stephenplusplus:spp--1354 into 009bc99 on GoogleCloudPlatform:master.