Fix compiler error about IDictionary 

```
$ haxe dotnet/system/collections/generic/IDictionary.hx -cs cs -D dll
dotnet/system/collections/generic/IDictionary.hx:4: lines 4-13 : Field Add has different type than in dotnet.system.collections.generic.ICollection
dotnet/system/collections/generic/IDictionary.hx:4: lines 4-13 : key : dotnet.system.collections.generic.IDictionary.TKey -> value : dotnet.system.collections.generic.IDictionary.TValue -> Void should be item : dotnet.system.collections.generic.KeyValuePair<dotnet.system.collections.generic.IDictionary.TKey, dotnet.system.collections.generic.IDictionary.TValue> -> Void
dotnet/system/collections/generic/IDictionary.hx:4: lines 4-13 : Field Remove has different type than in dotnet.system.collections.generic.ICollection
dotnet/system/collections/generic/IDictionary.hx:4: lines 4-13 : key : dotnet.system.collections.generic.IDictionary.TKey -> Bool should be item : dotnet.system.collections.generic.KeyValuePair<dotnet.system.collections.generic.IDictionary.TKey, dotnet.system.collections.generic.IDictionary.TValue> -> Bool
dotnet/system/collections/generic/IDictionary.hx:4: lines 4-13 : dotnet.system.collections.generic.IDictionary.TKey should be dotnet.system.collections.generic.KeyValuePair<dotnet.system.collections.generic.IDictionary.TKey, dotnet.system.collections.generic.IDictionary.TValue>
```