Andrew Childs f11611c878 json_writer: fix inverted sense in isAnyCharRequiredQuoting (#1120)
This bug is only affects platforms where `char` is unsigned.

When char is a signed type, values >= 0x80 are also considered < 0,
and hence require escaping due to the < ' ' condition.

When char is an unsigned type, values >= 0x80 match none of the
conditions and are considered safe to emit without escaping.

This shows up as a test failure:

* Detail of EscapeSequenceTest/writeEscapeSequence test failure:
/build/source/src/test_lib_json/main.cpp(3370): expected == result
  Expected: '["\"","\\","\b","\f","\n","\r","\t","\u0278","\ud852\udf62"]
  '
  Actual  : '["\"","\\","\b","\f","\n","\r","\t","ɸ","𤭢"]
  '
2019-12-28 15:04:24 +08:00
..