title: Parsing enum with EnumJsonAdapter doesn't work
body: <p>I have a data class User :</p>
  
  <pre><code> @JsonClass(generateAdapter = true)
  data class User(val accountName: String,
                  val gender: Gender,
                  val title: Title? = null)
  )
  </code></pre>
  
  <p>where gender and title must have only some values which are written in enums :</p>
  
  <pre><code>enum class Gender(val value: Int) {
    MALE(0), FEMALE(1), UNSPECIFIED(-1)
  }
  
  enum class Title(val value: String) {
    MRS("Mrs"), MS("Ms")
  }
  </code></pre>
  
  <p>I added <code>EnumJsonAdapter</code> to moshi builder, but it doesn't work. </p>
  
  <pre><code>fun fromJson(json: String): User? {
        val builder = Moshi.Builder()
            .add(Title::class.java, EnumJsonAdapter.create&lt;Title&gt;(Title::class.java).withUnknownFallback(null))
            .add(Gender::class.java, EnumJsonAdapter.create&lt;Gender&gt;(Gender::class.java).withUnknownFallback(Gender.UNSPECIFIED))
            .build()
        val jsonAdapter = builder.adapter(User::class.java)
  
        return jsonAdapter.fromJson(json)
      }
  
      fun toJson(obj: User): String {
        val builder = Moshi.Builder()
            .add(Title::class.java, EnumJsonAdapter.create&lt;Title&gt;(Title::class.java).withUnknownFallback(null))
            .add(Gender::class.java, EnumJsonAdapter.create&lt;Gender&gt;(Gender::class.java).withUnknownFallback(Gender.UNSPECIFIED))
            .build()
        val jsonAdapter = builder.adapter(User::class.java).serializeNulls()
  
        return jsonAdapter.toJson(obj)
      }
  </code></pre>
  
  <p>I receive title = "Mr" and it is parsed as null. On gender, I get this exception :</p>
  
  <pre><code>com.squareup.moshi.JsonDataException: Expected a string but was NUMBER at path $.data.gender
          at com.squareup.moshi.adapters.EnumJsonAdapter.fromJson(EnumJsonAdapter.java:93)
  </code></pre>
  
  <p>Is it something I am doing wrong with <code>EnumJsonAdapter</code> ? So what I see, it can not be used for int, but why it doesn't work ok for title which is a String ?</p>
  
  <p>If I use custom adapters for these enums everything works fine.     </p>