Do not throw ArgumenNullException in ConfigSettingEntry.GetAcceptedValues (#89)

This PR removes the ArgumenNullException present in ConfigSettingEntry.GetAcceptedValues(AcceptableValueBase values), enforcing the use of both MinValue and MaxValue for custom range classes (see issue's comments for a in-depth explanation)

Fixes #88

Author: MrAgeo

ConfigurationManager.Shared/ConfigSettingEntry.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
@@ -51,10 +51,9 @@ private void GetAcceptableValues(AcceptableValueBase values)
             else
             {
                 var minProp = t.GetProperty(nameof(AcceptableValueRange<bool>.MinValue), BindingFlags.Instance | BindingFlags.Public);
-                if (minProp != null)
+                var maxProp = t.GetProperty(nameof(AcceptableValueRange<bool>.MaxValue), BindingFlags.Instance | BindingFlags.Public);
+                if (minProp != null && maxProp != null)
                 {
-                    var maxProp = t.GetProperty(nameof(AcceptableValueRange<bool>.MaxValue), BindingFlags.Instance | BindingFlags.Public);
-                    if (maxProp == null) throw new ArgumentNullException(nameof(maxProp));
                     AcceptableValueRange = new KeyValuePair<object, object>(minProp.GetValue(values, null), maxProp.GetValue(values, null));
                     ShowRangeAsPercent = (AcceptableValueRange.Key.Equals(0) || AcceptableValueRange.Key.Equals(1)) && AcceptableValueRange.Value.Equals(100) ||
                                          AcceptableValueRange.Key.Equals(0f) && AcceptableValueRange.Value.Equals(1f);