fixed typo

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Human JavaScript is the best way to learn modern
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[fix] reject with "username already exists" error on username conflicts

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This commit was sponsored by Human JavaScript.
Human JavaScript is the best way to learn modern
web application development today, read it now:

http://go.hood.ie/human-javascript
Merge pull request #174 from hoodiehq/174_signup_error_fix

hoodie.account.signUp() -> 409 should include a message of “username already exists”