WIP: Use UnitSystem.DefaultCulture in ToString()

As fallback if no culture was specified, instead of 'null' that falls
back to the system default, which may be different if someone
explicitly set a different value in UnitSystem.DefaultCulture.

Expose DefaultCulture to WinRTC target as internal so we can
apply same behavior there (can't be public in WinRTC).

Author: angularsen

UnitsNet/CustomCode/UnitSystem.cs

@@ -141,7 +141,7 @@ public UnitSystem() : this(DefaultCulture)
         /// </summary>
 #if WINDOWS_UWP
         // Windows Runtime Component does not support exposing the IFormatProvider type in public API
-        private
+        internal
 #else
         public
 #endif

UnitsNet/GeneratedCode/Quantities/Mass.g.cs

@@ -1326,11 +1326,11 @@ public string ToString(MassUnit unit, [CanBeNull] Culture culture, [NotNull] str
             if (format == null) throw new ArgumentNullException(nameof(format));
             if (args == null) throw new ArgumentNullException(nameof(args));
 
-        // Windows Runtime Component does not support CultureInfo type, so use culture name string for public methods instead: https://msdn.microsoft.com/en-us/library/br230301.aspx
+            // Windows Runtime Component does not support CultureInfo or IFormatProvider types, so use culture name string for public methods instead: https://msdn.microsoft.com/en-us/library/br230301.aspx
 #if WINDOWS_UWP
-            IFormatProvider formatProvider = culture == null ? null : new CultureInfo(culture);
+            IFormatProvider formatProvider = culture == null ? UnitSystem.DefaultCulture : new CultureInfo(culture);
 #else
-            IFormatProvider formatProvider = culture;
+            IFormatProvider formatProvider = culture ?? UnitSystem.DefaultCulture;
 #endif
             double value = As(unit);
             object[] formatArgs = UnitFormatter.GetFormatArgs(unit, value, formatProvider, args);