Enable parameter modifier on extension methods

[zspitz]

Version Used:
C# 6.0

Steps to Reproduce:

public static class StringExtensions1 {
    //Doesn't compile -- "The parameter modifier cannot be used with 'this'"
    public static void Fill(this ref string s) {
        if (s == null) {s="";}
    }
}
public static class StringExtensions2 {
    //Doesn't compile -- "Do not use 'System.Runtime.CompilerServices.ExtensionAttribute'. Use 'this' keyword instead"
    [Extension] public static void Fill(ref string s, int count) {
        if (s == null) {s="";}
    }
}

Additional example with auto-initializing dictionary.

Expected Behavior:
Either syntax should compile and run.

Actual Behavior:
Neither syntax compiles.

[svick]

This is a duplicate of #165.

[zspitz]

@svick I don't know how I missed that.