Access Inflector's ivar only if it's defined
Access Inflector's ivar only if it's defined

After this commit ROM is free of warnings so `--warnings` should be enabled.

This commit eliminates:

    rom/lib/rom/support/inflector.rb:42: warning: instance variable @inflector not initialized

rom-rb/rom,endash/rom,pxlpnk/rom,jeremyf/rom,cored/rom,rom-rb/rom,rom-rb/rom,pvcarrera/rom,Snuff/rom,dcarral/rom,dekz/rom,denyago/rom,kwando/rom,vrish88/rom