Control flow characters not in [0x0000, 0x001F]

[marten-voorberg]

The following was also said in the offical repo issue. Someone should maybe look into this and add these tests if they are judged to be meaningful.

@marten-voorberg, maybe it would also be interesting to add unit tests for the following:
U+2028 and U+2029 handling
ECMAScript considers them line terminators but JSON does not. So ""\u2028\u2029"" should be considered valid JSON, and "[\u2028]" respectively "[\u2029]" should be considered invalid (all written as Java string literals here).
Other control characters handling
There are actually more control characters than defined in the JSON specification. However, since the JSON specification does not list them, they should be allowed unescaped in string values. For example the following should be considered valid JSON: ""\u007F\u009F""