Prefer to overwrite default service request timeout

[urjitbhatia]

Extend will honour values right to left, ie, it will overwrite values provided on the right. The current implementation ignores the reqopts.timeout in preference of the timeout set on the service itself which means users of this interface cannot set a timeout properly per request.

Example:

> extend({foo:1}, {bar:2}, {foo:3})
{ foo: 3, bar: 2 }

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Fixes googleapis#772 🦕