Added a method for verifying if the input has been provided with value

Hello,

I was designing an API and needed to support partial resource updates (changing only the fields which have been passed by the client, the rest should remain unchanged). 
Since in this case all the input objects in my input filter are optional it is useful to know which one of them have been provided with value. 

Thank you

Nikola
`Docbloc`s are helpful with param/return `type`s