fix bug(path through invalid signature)

For example: if the valid signature is ZjG7dBYj87Gn0DdJI69J6ZLIOCM=,
when ngx_access_token receives the signature ZjG7dBYj87Gn0DdJI69J6ZLIOCM=bokko
ngx_access_token manage to treat it as valid signature.

If signature is for example ZjG7dBYj87Gn0DdJI69J6ZLIOCM=bokko