Fix incorrect content length for the instrumented source

* The original code  used String#length, which returns the length
  of the string in characters, but the header must be the length of
  the string in bytes. This was causing errors because the browser
  was (presumably) truncating the content after receiving a (lower)
  specific number of bytes than the actual number of bytes in the
  content.
Merge pull request #51 from dkubb/multibyte-source-instrumentation

Fix incorrect content length for the instrumented source