support UTF-8 in old Writers

We had already fixed Value to hold UTF-8 properly, but only the newer
StreamWriter was writing UTF-8 properly.

Old FasterWriter etc. were using asCString() instead of asString() in
Value::writeValue().

Hopefully this change does not break any existing code. Seems unlikely.

issue #240

src/lib_json/json_writer.cpp

diff --git a/src/lib_json/json_writer.cpp b/src/lib_json/json_writer.cpp
index ef976bf..66ca217 100644
--- a/src/lib_json/json_writer.cpp
+++ b/src/lib_json/json_writer.cpp
@@ -317,8 +317,14 @@
     document_ += valueToString(value.asDouble());
     break;
   case stringValue:
-    document_ += valueToQuotedString(value.asCString());
+  {
+    // Is NULL possible for value.string_?
+    char const* str;
+    char const* end;
+    bool ok = value.getString(&str, &end);
+    if (ok) document_ += valueToQuotedStringN(str, static_cast<unsigned>(end-str));
     break;
+  }
   case booleanValue:
     document_ += valueToString(value.asBool());
     break;
@@ -382,7 +388,7 @@
     break;
   case stringValue:
   {
-    // Is NULL is possible for value.string_?
+    // Is NULL possible for value.string_?
     char const* str;
     char const* end;
     bool ok = value.getString(&str, &end);
@@ -599,8 +605,15 @@
     pushValue(valueToString(value.asDouble()));
     break;
   case stringValue:
-    pushValue(valueToQuotedString(value.asCString()));
+  {
+    // Is NULL possible for value.string_?
+    char const* str;
+    char const* end;
+    bool ok = value.getString(&str, &end);
+    if (ok) pushValue(valueToQuotedStringN(str, static_cast<unsigned>(end-str)));
+    else pushValue("");
     break;
+  }
   case booleanValue:
     pushValue(valueToString(value.asBool()));
     break;