Adding support for blank value on resize/fitin and improving filter input

-On Thumbor Server you can set blank values to "Resize" and "Fitin" (100x or x100). So I decided to implement this option on your lib.

-When you need to use the same Thumbor instance (Secret + Server) to create other urls, it makes copies of the same filter in the Url (/fill(white):fill(white):fill(white)).

I decided to create a check on the filtersCalls push. In this way, we don't need to create another Thumbor instance to use the same filters with another image url.