switched from YAML.safe_load* to SafeYAML.load*
added a test that YAML.load doesn't get clobbered

I THINK this is a good idea? I considered multiple approaches to testing this;
what I like about this dumb way (just try to deserialize a symbol) is that it's
nice and simple.
Merge pull request #1982 from dtao/safe-yaml-no-clobber