Fix Gem::BUNDLED_GEMS.find_gem return value

If the required name is different from the found gem name, return the
gem name, instead of true that means the required name is an exact gem
name.

lib/bundled_gems.rb

@@ -59,7 +59,7 @@ def self.find_gem(path)
     else
       return
     end
-    EXACT[n] or PREFIXED[n[%r[\A[^/]+(?=/)]]]
+    EXACT[n] or PREFIXED[n = n[%r[\A[^/]+(?=/)]]] && n
   end
 
   def self.warning?(name)