Fix consistent with the demo in JSON RFC

[XinShuoWang]

Although this is defined in the JSON RFC: The hexadecimal letters A through F can be uppercase or lowercase, many implementations use the uppercase solution. Would it be better to change it to uppercase?

[7].  Strings

   The representation of strings is similar to conventions used in the C
   family of programming languages.  A string begins and ends with
   quotation marks.  All Unicode characters may be placed within the
   quotation marks, except for the characters that MUST be escaped:
   quotation mark, reverse solidus, and the control characters (U+0000
   through U+001F).

   Any character may be escaped.  If the character is in the Basic
   Multilingual Plane (U+0000 through U+FFFF), then it may be
   represented as a six-character sequence: a reverse solidus, followed
   by the lowercase letter u, followed by four hexadecimal digits that
   encode the character's code point.  The hexadecimal letters A through
   F can be uppercase or lowercase.  So, for example, a string
   containing only a single reverse solidus character may be represented
   as "\u005C".