Added default parameter values to IMaybe implementations

- while strictly speaking not necessary, they make it more clear to read

Author: lord-executor

src/MaybeSharp/Maybe.cs

@@ -64,7 +64,7 @@ public IMaybe<TResult> Bind<TResult>(Func<T, IMaybe<TResult>> just) where TResul
 				return just(_value);
 			}
 
-			public IMaybe<T> Default(Func<IMaybe<T>> defaultValue)
+			public IMaybe<T> Default(Func<IMaybe<T>> defaultValue = null)
 			{
 				return this;
 			}
@@ -111,7 +111,7 @@ public IMaybe<TResult> Bind<TResult>(Func<T, IMaybe<TResult>> just) where TResul
 				return Nothing<TResult>();
 			}
 
-			public IMaybe<T> Default(Func<IMaybe<T>> defaultValue)
+			public IMaybe<T> Default(Func<IMaybe<T>> defaultValue = null)
 			{
 				return defaultValue();
 			}