dereference btoa before encoding

In Chrome, initial setup showed an Uncaught TypeError: Illegal Invocation Error at this point.

Not sure why it wasn't working, but after dereferencing, it now encodes just fine.

Related to:
wordnik/swagger-js#63
wordnik/swagger-js#64
Merge pull request #86 from MaxPMagee/master

btoa was not working before it was dereferenced