Prevent ObjC generator from producing invalid var names.

I was running into trouble with header parameters, since things like `X-Parameter-Name` are not valid Objective-C identifiers.

This fix was lifted from [the Java generator](https://github.com/wordnik/swagger-codegen/blob/c8c59fe93a14644dbdd2bc15910ea6a16edddc98/src/main/scala/com/wordnik/swagger/codegen/BasicJavaGenerator.scala#L106).