Prevent malformed classnames.

In the event that the provided printer's class name has "Provider"; don't try forcing "Printer" on to it; or you'd get RubyProf::GraphPrinterPrinter. This is a light fix for the attached:
![snapshot63](https://f.cloud.github.com/assets/452100/1598358/55a958b6-532d-11e3-8482-44b5e644899f.png)