Added default user agent string to WebClient.
Added default user agent string to WebClient.

yanlinlin82/gurtle,ramkrishanbhatt/gurtle,yanlinlin82/gurtle,yanlinlin82/gurtle,zephyrer/gurtle,ramkrishanbhatt/gurtle,zephyrer/gurtle,yanlinlin82/gurtle,ramkrishanbhatt/gurtle,zephyrer/gurtle