A couple benchmark fixes

- the wrong benchmark function was being called in a couple spots.

- fix function names in a comment.
Merge pull request #18 from bmatsuo/benchmark-fixes

A couple benchmark fixes
fix error handling bugs in reader.WriteTo

- r.err is checked before r.WriteTo begins writing. This avoids writing
  corrupt data to the io.Writer.

- r.err is set when decoding errors occur in r.WriteTo. A copyWriter
  type is introduced to let r.WriteTo distinguish decoding errors vs
  write errors and allow reading to safely resume after a write error
  occurs.
rename copyWriter to bufferFallbackWriter -- clarify some docs
inline bufferFallbackReader construction