Fix result_type issues

[dkarrasch]

Fixes #210, fixes #209.

It seems very hard to include a generic fallback à la result_type(...) = Float64 again. So, the fix to #209 is to handle the Missing case explicitly, and the fix to #210 is to insist that a result_type method be given for "exotic" metrics that cannot handle Number arguments, and so we do it for Haversine and Spherical_Angle.