Fix HOTP calculate value length less then require digit

According to [ rfc4226](http://tools.ietf.org/html/rfc4226) specification 5.3, Digit = 7 or more SHOULD be considered in order to extract a longer HOTP value. But we fail to do this, for example:  

```
speakeasy.hotp {key: 'PV5H2NBQEZ4XGU3RPAZUSOLEPVZCIR3OMMXWQPTQN4ZT6UCMHI7Q', counter: 46061050, length: 8, encoding: 'base32'}  
```
not  return '08197819', it return '9' .