Add some "app info" tests for `X-Stripe-Client-User-Agent`
Add some "app info" tests for `X-Stripe-Client-User-Agent`

The current tests for "app info" test against `User-Agent`, but don't
test against the `X-Stripe-Client-User-Agent` header. This patch adds a
few extra tests for safety.

Related: #607.

ChannelMeter/stripe-go,stripe/stripe-go,ChannelMeter/stripe-go,stripe/stripe-go