added 'json' filter

src/main/java/liqp/filters/Filters.java

@@ -63,7 +63,8 @@ public final class Filters {
             new Upcase(), //
             new Url_Decode(), //
             new Url_Encode(), //
-            new Where() //
+            new Where(), //
+            new Json() //
     );
 
     static Filters JEKYLL_EXTRA_FILTERS = Filters.of( //

src/main/java/liqp/filters/Json.java

@@ -0,0 +1,23 @@
+package liqp.filters;
+
+import com.fasterxml.jackson.core.JsonProcessingException;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import liqp.TemplateContext;
+import liqp.TemplateParser;
+
+public class Json extends Filter {
+    @Override
+    public Object apply(Object value, TemplateContext context, Object... params) {
+        ObjectMapper mapper = context.getParser().mapper;
+
+        try {
+            return mapper.writeValueAsString(value);
+        } catch (JsonProcessingException e) {
+            context.addError(e);
+            if (context.getErrorMode() == TemplateParser.ErrorMode.STRICT) {
+                throw new RuntimeException(e.getMessage(), e);
+            }
+            return value;
+        }
+    }
+}

src/test/java/liqp/filters/JsonTest.java

@@ -0,0 +1,31 @@
+package liqp.filters;
+
+import liqp.Template;
+import liqp.TemplateParser;
+import org.junit.Test;
+
+import java.util.HashMap;
+
+import static org.junit.Assert.*;
+
+public class JsonTest {
+
+    @Test
+    public void testWhenStringIsInputShouldBeStringified() {
+        Template template = TemplateParser.DEFAULT.parse("{{ 'Hello, World!' | json }}");
+        String rendered = template.render();
+        assertEquals("\"Hello, World!\"", rendered);
+    }
+
+    @Test
+    public void testWhenObjectIsInputShouldBeStringified() {
+        Template template = TemplateParser.DEFAULT.parse("{{ obj | json }}");
+        java.util.Map<String, Object> map = new HashMap<>();
+        java.util.Map<String, Object> nested = new HashMap<>();
+        nested.put("key", "value");
+        map.put("obj", nested);
+
+        String rendered = template.render(map);
+        assertEquals("{\"key\":\"value\"}", rendered);
+    }
+}