Skip nil tokens in RDoc::TokenStream#tokens_to_s.

For reasons I don't know, `token_stream` sometimes contain `nil` elements.
This is handled in `RDoc::TokenStream.to_html` but not in
`RDoc::TokenStream#tokens_to_s`. This adds handling to
`RDoc::TokenStream#tokens_to_s`.
Merge pull request #318 from calleerlandsson/fix-tokens-to-s

Skip nil tokens in RDoc::TokenStream#tokens_to_s.