Pre-fill argument if content argument is defined

[s2b]

Fluid should pre-fill $this->arguments['myArgument'] in ViewHelpers if getContentArgumentName() is set to myArgument and the argument is supplied via tag content. Currently, it is the other way around: $this->renderChildren() uses $this->arguments['myArgument'] as fallback. Both variants should be in sync, so ViewHelpers can just use $this->arguments and define the argument as required.