Added the missing $() convenience method for messaging contracts (#623)

Fixes gh-619

spring-cloud-contract-spec/src/main/groovy/org/springframework/cloud/contract/spec/internal/Input.groovy

@@ -95,6 +95,10 @@ class Input extends Common {
 		return new DslProperty(client.clientValue, clientValue)
 	}
 
+	DslProperty $(ClientDslProperty client) {
+		return value(client)
+	}
+
 	@EqualsAndHashCode(includeFields = true, callSuper = true)
 	@ToString(includeSuper = true)
 	static class BodyType extends DslProperty {

spring-cloud-contract-spec/src/main/groovy/org/springframework/cloud/contract/spec/internal/OutputMessage.groovy

@@ -97,6 +97,10 @@ class OutputMessage extends Common {
 		return new DslProperty(value, server.serverValue)
 	}
 
+	DslProperty $(ServerDslProperty server) {
+		return value(server)
+	}
+
 	/**
 	 * @deprecated Deprecated in favor of bodyMatchers to support other future bodyMatchers too
 	 */

spring-cloud-contract-spec/src/test/groovy/org/springframework/cloud/contract/spec/internal/InputSpec.groovy

@@ -0,0 +1,20 @@
+package org.springframework.cloud.contract.spec.internal
+
+import spock.lang.Specification
+/**
+ * @author Tim Ysewyn
+ */
+class InputSpec extends Specification {
+
+	def 'should set property when using the $() convenience method'() {
+		given:
+			Input input = new Input()
+			DslProperty property
+		when:
+			input.with {
+				property = $(consumer(regex("[0-9]{5}")))
+			}
+		then:
+			(property.serverValue as String).matches(/[0-9]{5}/)
+	}
+}

spring-cloud-contract-spec/src/test/groovy/org/springframework/cloud/contract/spec/internal/OutputMessageSpec.groovy

@@ -0,0 +1,20 @@
+package org.springframework.cloud.contract.spec.internal
+
+import spock.lang.Specification
+/**
+ * @author Tim Ysewyn
+ */
+class OutputMessageSpec extends Specification {
+
+	def 'should set property when using the $() convenience method'() {
+		given:
+			Input input = new Input()
+			DslProperty property
+		when:
+			input.with {
+				property = $(consumer(regex("[0-9]{5}")))
+			}
+		then:
+			(property.serverValue as String).matches(/[0-9]{5}/)
+	}
+}