fix UserAgent Header encoding

Hello.
I was not able to create any request getting this error:

"Specified value has invalid Control characters.
Parameter name: value"

Thats how my UA header looks like: 
"PayPalSDK/paypal-core-dotnet 1.4.1 ;lang=DOTNET;v=4.0.30319.18034;bit=32;os=Microsoft Windows 7 Максимальная  6.1.7601.65536;"