Support aggregation operators $acos and $acosh via expression method reference.

This commit registers the acos(...) and acosh(...) methods for transformation via SpEL.

Original Pull Request: #3858

Author: christophstrobl

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/spel/MethodReferenceNode.java

@@ -99,6 +99,8 @@ public class MethodReferenceNode extends ExpressionNode {
 		map.put("asinh", singleArgRef().forOperator("$asinh"));
 		map.put("cos", singleArgRef().forOperator("$cos"));
 		map.put("cosh", singleArgRef().forOperator("$cosh"));
+		map.put("acos", singleArgRef().forOperator("$acos"));
+		map.put("acosh", singleArgRef().forOperator("$acosh"));
 		map.put("tan", singleArgRef().forOperator("$tan"));
 		map.put("tanh", singleArgRef().forOperator("$tanh"));
 		map.put("rand", emptyRef().forOperator("$rand"));

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/SpelExpressionTransformerUnitTests.java

@@ -1099,6 +1099,16 @@ void shouldRenderCosh() {
 		assertThat(transform("cosh(angle)")).isEqualTo("{ \"$cosh\" : \"$angle\"}");
 	}
 
+	@Test // GH-3707
+	void shouldRenderACos() {
+		assertThat(transform("acos(angle)")).isEqualTo("{ \"$acos\" : \"$angle\"}");
+	}
+
+	@Test // GH-3707
+	void shouldRenderACosh() {
+		assertThat(transform("acosh(angle)")).isEqualTo("{ \"$acosh\" : \"$angle\"}");
+	}
+
 	@Test // GH-3730
 	void shouldRenderTan() {
 		assertThat(transform("tan(angle)")).isEqualTo("{ \"$tan\" : \"$angle\"}");