Fix #5261 Don't escape Unicode characters in PSON

This patch removes the escaping of valid UTF-8 sequences as "\uXXXX".
This code was unreliable, as it relied on Iconv's ability to convert
those codepoints between UTF-8 and UTF-16, but some versions of Iconv
barf on some valid codepoints.

Invalid UTF-8 sequences are still passed through unchanged. We believe
that this is fine; if you are concerned about complience with the JSON
standard, what we are doing is equivalent to:

• interpreting binary files as Latin-1 encoded character sequences
• JSON-encoding those characters according to RFC 4627
• outputting the JSON as Latin-1

This allows all raw binary files to be transmitted losslessly.

Paired-With: Paul Berry <paul@puppetlabs.com>