Fixed invalid OptionalProperty conversion

without this change a toString was executed on an optional property due to a missing method that parses those kind of props.
with this change we've added a missing method and also we've made optional property toString return the proper pattern

fixes #173

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

@@ -47,6 +47,6 @@ class ExecutionProperty {
 
 	@Override
 	public String toString() {
-		return executionCommand;
+		return executionCommand
 	}
 }

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

@@ -38,4 +38,10 @@ class OptionalProperty {
 	String optionalPattern() {
 		return "($value)?"
 	}
+
+
+	@Override
+	public String toString() {
+		return optionalPattern()
+	}
 }