We use following algorithm to select protocol:
1. If server's list contains "spdy/2", this function selects
"spdy/2" and returns 1. The following steps are not taken.
2. If server's list contains "http/1.1", this function selects
"http/1.1" and returns 0. The following step is not taken.
3. This function selects "spdy/2" and returns -1. (So called
non-overlap case).
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