miniscore _.defaults works bad with nulls

[0xF013]

https://github.com/weluse/hyperagent/blob/master/lib/hyperagent/miniscore.js#L76
this line causes null properties returned after a forces fetch to not overwrite the old value. Lodash's _.defaults only checks for undefined and has nulls as acceptable values

[passy]

@0xF013 Cheers, underscore does the same thing. I'll update it accordingly.