Add arity-checking anywhere it is missing

[headius]

As part of jruby/jruby#7751 we are recommending that all extension code manually check the arity of incoming arguments to variable- arity methods, as in CRuby. This ensures that all call paths will be checked, including direct paths from Java or invokedynamic, and avoids array indexing errors in these situations.

[headius]

@hsbt @jeremyevans @mrkn If we can get this in a small release that would be great. Let me know if you can do it or if I should.

[jeremyevans]

I've never done a stringio release. We should probably ask @nobu, since he is the maintainer and did the last release.

[kou]

I can do it.

[kou]

@headius Released!