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/codegen/src/main/java/com/squareup/moshi/kotlin/codegen/api/PropertyGenerator.kt

@@ -26,7 +26,7 @@ internal class PropertyGenerator(
   val isTransient: Boolean = false
 ) {
   val name = target.name
-  val jsonName = target.jsonName
+  val jsonName = target.jsonName ?: target.name
   val hasDefault = target.hasDefault
 
   lateinit var localName: String

kotlin/codegen/src/main/java/com/squareup/moshi/kotlin/codegen/api/TargetParameter.kt

@@ -22,6 +22,6 @@ internal data class TargetParameter(
   val name: String,
   val index: Int,
   val hasDefault: Boolean,
-  val jsonName: String = name,
+  val jsonName: String? = null,
   val qualifiers: Set<AnnotationSpec>? = null
 )

kotlin/codegen/src/main/java/com/squareup/moshi/kotlin/codegen/api/TargetProperty.kt

@@ -24,7 +24,7 @@ internal data class TargetProperty(
   val propertySpec: PropertySpec,
   val parameter: TargetParameter?,
   val visibility: KModifier,
-  val jsonName: String
+  val jsonName: String?
 ) {
   val name: String get() = propertySpec.name
   val type: TypeName get() = propertySpec.type

kotlin/codegen/src/main/java/com/squareup/moshi/kotlin/codegen/metadata.kt

@@ -77,7 +77,7 @@ internal fun primaryConstructor(kotlinApi: TypeSpec, elements: Elements): Target
         index = index,
         hasDefault = parameter.defaultValue != null,
         qualifiers = parameter.annotations.qualifiers(elements),
-        jsonName = parameter.annotations.jsonName() ?: name.escapeDollarSigns()
+        jsonName = parameter.annotations.jsonName()
     )
   }