Make string to double conversion locale invariant.

Applying Stephen Wagner's fix from this commit to JsonCpp 1.9.5: c36b636

Author: mike-malburg

src/lib_json/json_reader.cpp

@@ -601,6 +601,7 @@ bool Reader::decodeDouble(Token& token, Value& decoded) {
   double value = 0;
   String buffer(token.start_, token.end_);
   IStringStream is(buffer);
+  is.imbue( std::locale::classic() );
   if (!(is >> value)) {
     if (value == std::numeric_limits<double>::max())
       value = std::numeric_limits<double>::infinity();