correct uaa access_token err when token refresh

When token expired, browsing instance dashboard failed.
Following the code, it is the UaaSession.build refresh the token_info.
```
Before refresh, the access_token is symbol.
#<CF::UAA::TokenInfo:0x007f69fed46380 @info={:access_token=>"eyJhbGciOiJSUzI1
After refresh, the access_token is string.
#<CF::UAA::TokenInfo:0x007f69fed5e840 @info={"access_token"=>"eyJhbGciOiJ
The type of @info's key is changed. But the method access_token just return symbol type.
```
The pr reference cf-uaa-lib
https://github.com/cloudfoundry/cf-uaa-lib/blob/master/lib/uaa/token_issuer.rb#L38

the internal issue link: http://rnd-github.huawei.com/paas/cf-mysql-broker/issues/1