add unescape_html filter

src/main/java/com/hubspot/jinjava/lib/filter/UnescapeHtmlFilter.java

@@ -0,0 +1,46 @@
+/**********************************************************************
+ * Copyright (c) 2022 HubSpot Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ **********************************************************************/
+package com.hubspot.jinjava.lib.filter;
+
+import com.hubspot.jinjava.doc.annotations.JinjavaDoc;
+import com.hubspot.jinjava.doc.annotations.JinjavaParam;
+import com.hubspot.jinjava.doc.annotations.JinjavaSnippet;
+import com.hubspot.jinjava.interpret.JinjavaInterpreter;
+import java.util.Objects;
+import org.apache.commons.lang3.StringEscapeUtils;
+
+@JinjavaDoc(
+  value = "Converts HTML entities in string s to Unicode characters.",
+  input = @JinjavaParam(value = "s", desc = "String to unescape", required = true),
+  snippets = {
+    @JinjavaSnippet(
+      code = "{% set escaped_string = \"<div>This &amp; that</div>\" %}\n" +
+      "{{ escaped_string|unescape_html }}"
+    )
+  }
+)
+public class UnescapeHtmlFilter implements Filter {
+
+  @Override
+  public Object filter(Object object, JinjavaInterpreter interpreter, String... arg) {
+    return StringEscapeUtils.unescapeHtml4(Objects.toString(object, ""));
+  }
+
+  @Override
+  public String getName() {
+    return "unescape_html";
+  }
+}

src/test/java/com/hubspot/jinjava/lib/filter/UnescapeHtmlFilterTest.java

@@ -0,0 +1,25 @@
+package com.hubspot.jinjava.lib.filter;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import com.hubspot.jinjava.BaseInterpretingTest;
+import org.junit.Before;
+import org.junit.Test;
+
+public class UnescapeHtmlFilterTest extends BaseInterpretingTest {
+  UnescapeHtmlFilter f;
+
+  @Before
+  public void setup() {
+    f = new UnescapeHtmlFilter();
+  }
+
+  @Test
+  public void itUnescapes() {
+    assertThat(f.filter("", interpreter)).isEqualTo("");
+    assertThat(f.filter("me & you", interpreter)).isEqualTo("me & you");
+    assertThat(f.filter("jeff's & jack's bogüs journey", interpreter))
+      .isEqualTo("jeff's & jack's bogüs journey");
+    assertThat(f.filter(1, interpreter)).isEqualTo("1");
+  }
+}