JSON: Add toString() method for StringPairArray

[daleonov]

Had a few StringPairArray's I had to convert to JSON, was surprised JUCE can't do that yet. I used JSONFormatter stuff as much as possible, so it should behave nicely. Example:

    juce::StringPairArray data;

    DBG("*** Empty array ***");
    DBG("Single line" << juce::newLine << juce::JSON::toString(data, true));
    DBG("Multiple lines" << juce::newLine << juce::JSON::toString(data, false));

    DBG(juce::newLine << "*** One item ***");
    data.set("Key1", "Value1");
    DBG("Single line" << juce::newLine << juce::JSON::toString(data, true));
    DBG("Multiple lines" << juce::newLine << juce::JSON::toString(data, false));

    DBG(juce::newLine << "*** A bunch of items ***");
    data.set("Key2", "Value2");
    data.set("Key3", "Value\that\needs\escaping");
    DBG("Single line" << juce::newLine << juce::JSON::toString(data, true));
    DBG("Multiple lines" << juce::newLine << juce::JSON::toString(data, false));

Output:

*** Empty array ***
Single line
{}
Multiple lines
{}

*** One item ***
Single line
{"Key1": "Value1"}
Multiple lines
{
  "Key1": "Value1"
}

*** A bunch of items ***
Single line
{"Key1": "Value1", "Key2": "Value2", "Key3": "Value\\that\\needsescaping"}
Multiple lines
{
  "Key1": "Value1",
  "Key2": "Value2",
  "Key3": "Value\\that\\needsescaping"
}

It can also live as StringPairArray method, but I think you prefer to keep all JSON stuff in one class. Also, noticed weird string escaping behaviour (see how one backslash has disappeared in last item?), but that's another matter.