Add test for JSON encode and decode of "\a\b"

There was no test for the encoding to JSON of "\a\b" or the decoding of
the corresponding JSON data. This change adds such tests.
Fix Mojo::JSON - encode "\a\b"

Mojo::JSON has incorrect value associated with the escaped special
character 'b'. This change corrects the value, from '\x07' to '\x08'.
Merge pull request #561 from ig3/JSON_b

Mojo::JSON encoding and decoding of binary data "\a\b"