Add an EL implementation to the Undertow Starter

Unlike the Tomcat and Jetty starters, the Undertow starter does not
provide an EL implementation. This leads to failures when you try to use
Hibernate Validator with the Undertow starter.

To bring the Undertow starter into line with the other two embedded
container starters, this commit adds Glassfish’s EL implementation to
the Undertow starter. This is the implementation that’s used by the
Jetty starter. If/when Undertow provides JSP support and, therefore,
starts using EL itself, we should align with it.

Closes gh-1979

spring-boot-dependencies/pom.xml

@@ -62,6 +62,7 @@
 		<flyway.version>3.0</flyway.version>
 		<freemarker.version>2.3.21</freemarker.version>
 		<gemfire.version>7.0.2</gemfire.version>
+		<glassfish-el.version>3.0.0</glassfish-el.version>
 		<gradle.version>1.6</gradle.version>
 		<groovy.version>2.3.7</groovy.version>
 		<gson.version>2.3</gson.version>
@@ -950,6 +951,11 @@
 				<artifactId>freemarker</artifactId>
 				<version>${freemarker.version}</version>
 			</dependency>
+			<dependency>
+				<groupId>org.glassfish</groupId>
+				<artifactId>javax.el</artifactId>
+				<version>${glassfish-el.version}</version>
+			</dependency>
 			<dependency>
 				<groupId>org.glassfish.jersey.containers</groupId>
 				<artifactId>jersey-container-servlet</artifactId>

spring-boot-starters/spring-boot-starter-undertow/pom.xml

@@ -37,5 +37,9 @@
 			<groupId>javax.servlet</groupId>
 			<artifactId>javax.servlet-api</artifactId>
 		</dependency>
+		<dependency>
+			<groupId>org.glassfish</groupId>
+			<artifactId>javax.el</artifactId>
+		</dependency>
 	</dependencies>
 </project>