Buffer: also forward the to_s method

[mkauf]

Instead of actual data, the example program "client.rb" sent the string "#<HTTP2::Buffer:0x0056529cd1d1a8>" when I tried it. Probably this behavior depends on the Ruby version (I have tested with Ruby 2.3.1).

I don't know if there is a better bugfix, but also forwarding the "to_s" method fixes the problem.

[coveralls]

Coverage remained the same at 96.898% when pulling 4c2216e on mkauf:buffer_bugfix into c11d93a on igrigorik:master.

[igrigorik]

Some context on to_s vs to_str (which we already implement): https://gist.github.com/kinopyo/5682347

@georgeu2000 WDYT? I'll defer to you on this one.

[georgeu2000]

@igrigorik Regarding context link:

You should not implement to_str unless your object acts like a string

It seems to me that a buffer does act like a string, so it should implement to_str.

LGTM. Merging.

[mkauf]

Thank you for merging!