Fixed missing support for execute method in URL

without this change the execute method in URL is quoted and treated as string
with this change we're supporting it OOB

fixes #11

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

@@ -44,7 +44,7 @@ class ExecutionProperty {
 	}
 
 	@Override
-	public String toString() {
+	String toString() {
 		return executionCommand
 	}
 }