Made it possible to drop null placeholders from array output.

This can be used when it's clear that the consumer is able to deal with
this, as web browsers are. Thanks to Yatin Chawathe for the patch.

src/lib_json/json_writer.cpp

@@ -192,7 +192,8 @@ Writer::~Writer()
 // //////////////////////////////////////////////////////////////////
 
 FastWriter::FastWriter()
-   : yamlCompatiblityEnabled_( false )
+   : yamlCompatiblityEnabled_( false ),
+     dropNullPlaceholders_( false )
 {
 }
 
@@ -204,6 +205,13 @@ FastWriter::enableYAMLCompatibility()
 }
 
 
+void
+FastWriter::dropNullPlaceholders()
+{
+   dropNullPlaceholders_ = true;
+}
+
+
 std::string 
 FastWriter::write( const Value &root )
 {
@@ -220,7 +228,7 @@ FastWriter::writeValue( const Value &value )
    switch ( value.type() )
    {
    case nullValue:
-      document_ += "null";
+      if (!dropNullPlaceholders_) document_ += "null";
       break;
    case intValue:
       document_ += valueToString( value.asLargestInt() );

src/test_lib_json/main.cpp

@@ -1399,6 +1399,23 @@ ValueTest::checkIsEqual( const Json::Value &x, const Json::Value &y )
     JSONTEST_ASSERT( y.compare( x ) == 0 );
 }
 
+
+struct WriterTest : JsonTest::TestCase
+{
+};
+
+
+JSONTEST_FIXTURE( WriterTest, dropNullPlaceholders )
+{
+    Json::FastWriter writer;
+    Json::Value nullValue;
+    JSONTEST_ASSERT( writer.write(nullValue) == "null\n" );
+
+    writer.dropNullPlaceholders();
+    JSONTEST_ASSERT( writer.write(nullValue) == "\n" );
+}
+
+
 int main( int argc, const char *argv[] )
 {
    JsonTest::Runner runner;
@@ -1420,5 +1437,6 @@ int main( int argc, const char *argv[] )
    JSONTEST_REGISTER_FIXTURE( runner, ValueTest, compareArray );
    JSONTEST_REGISTER_FIXTURE( runner, ValueTest, compareObject );
    JSONTEST_REGISTER_FIXTURE( runner, ValueTest, compareType );
+   JSONTEST_REGISTER_FIXTURE( runner, WriterTest, dropNullPlaceholders );
    return runner.runCommandLine( argc, argv );
 }