Support as input multiple headers with the same key

[davideme]

Check test spdy3HeadersList

https://github.com/square/okhttp/blob/ee83d8d26afd92d27fbcd2a328e882f25a5090c6/okhttp-tests/src/test/java/okhttp3/internal/http/HeadersTest.java#L87

[davideme]

I think it's better to keep it for post 1.0