Properly Implement StringSegment and StringSegmentComparer GetHashCode

[Tratcher]

#213
StringSegment.GetHashCode and StringSegmentComparer.GetHashCode(StringSegment) both substring and then string.GetHashCode() which defeats the purpose of using StringSegment.

Note that .NET Core strings use randomized hash codes to mitigate issues like dictionary hash collision attacks.

The header parsers in HttpAbstractions call both of these methods the header value types GetHashCode implementations, but I'm not aware of any instances where those header value types are used as keys into hashed collections.

[aspnet-hello]

This issue was moved to aspnet/Home#2365