rename defaults.jl to fallbacks.jl

This fits the header description better, and is the same filename as used for Tables.jl

Author: visr

src/GeoInterface.jl

@@ -4,7 +4,7 @@ using Base.Iterators
 
 include("types.jl")
 include("interface.jl")
-include("defaults.jl")
+include("fallbacks.jl")
 include("utils.jl")
 
 export testgeometry