Added support for netstandard2.0 besides netstandard2.1 (multi target framework) to allow usage from UWP (netstandard 2.1 not available in UWP until .net 5)

MediaBrowser.Model/Extensions/StringHelper.cs

@@ -22,6 +22,11 @@ namespace MediaBrowser.Model.Extensions
                 return str;
             }
 
+#if NETSTANDARD2_0
+            char[] a = str.ToCharArray();
+            a[0] = char.ToUpperInvariant(a[0]);
+            return new string(a);
+#else
             return string.Create(
                 str.Length,
                 str,
@@ -33,6 +38,7 @@ namespace MediaBrowser.Model.Extensions
                         chars[i] = buf[i];
                     }
                 });
+#endif
         }
     }
 }