removed std::forward(args) from operator() methods

[tyhenry]

this fixes a bug where signal argument types with move c'tors (i.e. std::string, std::vector, etc.) would be invalidated by the first callback.

e.g.

signal<void(std::string)> signalStr;

int i = 0;
auto fn = [&i](std::string s) { std::cout << ++i << ": " << s << std::endl; };
signalStr.connect( fn );
signalStr.connect( fn );

std::string str = "hello";
signalStr( str );

/*

before fix:

1: hello
2: 

after fix:

1: hello
2: hello

*/