Avoid modifying the original request as per http.RoundTripper contract.
In UnauthenticatedRateLimitedTransport.RoundTrip, we need to modify the
URL of the request only, while in BasicAuthTransport.RoundTrip, we need
to modify only the headers.

Hence, we get rid of cloneRequest method which wasn't working good for
the needs of UnauthenticatedRateLimitedTransport.RoundTrip. Instead, now
we have the implementation of cloneRequest inline in both the RoundTrip
interfaces.

We decided to make the cloneRequest implementation inline because of its
use at only these two places and we think we won't gain much by making a
generic implementation of cloneRequest as a function. To follow the
discussion on this, check the GitHub issue -
google#556.

Fixes google#556

Change []string(nil) to []string{} to make it easier to understand
because the former isn't a common pattern in Go.