CLR Functions With Optional Parameters

I'm rewriting a T-SQL function that is called "Proper Case" which takes in a sentence and returns the sentence with the first letter of each word capitalized.

The new CLR(C#) function implements new functionality where it can take a string and turn it into a properly cased string, sentence cased string, lower cased, upper cased, and toggle cased.

The function takes in one parameter, the string, and the casing type, a string also. I want the casing type to be optional where if it's not passed in it defaults to "proper". The reason why I want this is so that I can easily replace the existing T-SQL function without having to add an extra parameter to each call to make life easier.

I know you can do it for SQL stored procedures, but I seem to be having trouble with the function.

Things I've tried:
1) Overloading the function in C# doesn't work because SQL functions don't like overloaded functions.

2) Attempting to set a "default" value to the parameter.
a) casingType nvarchar(4000) = 'proper'
b) default casingType nvarchar(4000) = 'proper'

Any ideas would be greatly appreciated. Thanks!