title: WebTestClient check that jsonPath contains sub string
body: <p>In MockMvc there is an ability to assert that jsonPath contains substing                </p>
  
  <pre><code>.andExpect(jsonPath("$.error.message")
  .value(containsString("message")))
  </code></pre>
  
  <p>I wonder if there a nice way to do the same for WebTestClient, the syntax is a bit different</p>
  
  <pre><code>webClient.post().uri("/test")
                  .contentType(MediaType.APPLICATION_JSON)
                  .body(Mono.just(MyRequest), MyRequest.class)
                  .exchange()
                  .expectStatus().isBadRequest()
                  .expectBody()
                  .jsonPath("$.message").isEqualTo("message")
  </code></pre>
  
  <p>But I've found only isEqualTo method that related to it.</p>
  
  <p>It could be done with exctracting getBodyAsString() from WebTestClient.BodyContentSpec but it doesn't look good.</p>