underscore classVarName to fix route issue in ROR (#706)

Author: wing328

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/RubyOnRailsServerCodegen.java

@@ -250,6 +250,16 @@ public String toApiFilename(String name) {
         return underscore(name) + "_controller";
     }
 
+    @Override
+    public String toApiVarName(String name) {
+        if (name.length() == 0) {
+            return "api";
+        }
+
+        // e.g. PhoneNumber => phone_number
+        return underscore(sanitizeName(name));
+    }
+
     @Override
     public String toApiName(String name) {
         if (name.length() == 0) {