First pass at property expansions

Expansions.java

/**
   * Returns a new {@link Dictionary} where values containing system property notation such as
   * {@code ${my.system.property.name}} are substituted with their appropriate values.
   */
  private Dictionary<String, Object> expand(Dictionary<String, Object> original) {
    Dictionary<String, Object> expanded = new Hashtable<>();
    for (String key : list(original.keys())) {
      Object val = original.get(key);
      if (val instanceof String) {
        expanded.put(
            key,
            Stream.of(val)
                .map(String.class::cast)
                .flatMap(PROP_PATTERN::splitAsStream)
                .map(this::substituteProperty)
                .collect(Collectors.joining()));
      } else {
        expanded.put(key, val);
      }
    }
    return expanded;
  }

  private String substituteProperty(String fragment) {
    if (PROP_PATTERN.matcher(fragment).matches()) {
      String propertyName = fragment.substring(2, fragment.length() - 1);
      String propertyValue = System.getProperty(propertyName);
      return (propertyValue == null) ? "" : propertyValue;
    }
    return fragment;
  }