Fix @field:Json parameters annotations always taking precedence (#946)

* Add regression test for field json annotations

* Make json name null if absent rather than default to name

This fixes incorrect behavior where the parameter json name would always win out over the property value

kotlin/tests/src/test/kotlin/com/squareup/moshi/kotlin/codegen/GeneratedAdaptersTest.kt

@@ -1126,14 +1126,32 @@ class GeneratedAdaptersTest {
   }
 
   @JsonClass(generateAdapter = true)
-  class InternalPropertyWithoutBackingField  {
+  class InternalPropertyWithoutBackingField {
 
     @Transient
     private var foo: Int = 5
 
     internal var bar
       get() = foo
-      set(f) { foo = f}
+      set(f) {
+        foo = f
+      }
+  }
+
+  @JsonClass(generateAdapter = true)
+  data class ClassWithFieldJson(
+      @field:Json(name = "_links") val links: String
+  ) {
+    @field:Json(name = "_ids") var ids: String? = null
+  }
+
+  // Regression test to ensure annotations with field site targets still use the right name
+  @Test fun classWithFieldJsonTargets() {
+    val moshi = Moshi.Builder().build()
+    val adapter = moshi.adapter<ClassWithFieldJson>()
+    //language=JSON
+    val instance = adapter.fromJson("""{"_links": "link", "_ids": "id" }""")!!
+    assertThat(instance).isEqualTo(ClassWithFieldJson("link").apply { ids = "id" })
   }
 }