Using the proxy settings from the configuration file while requesting OAuthTokenCredential.

Guess it would make sense to use the proxy configuration data configured in the properties file.
I was testing my implementation and was surprised that I was not able to connect to paypal. After debugging and digging in the sdk code I found that the proxy settings were completely ignored when requesting the OAuthToken.
I added the missing mapping step for the proxy settings in the OAuthTokenCredential class in order to be able to request a token when behind a proxy.