BookSource.kt: Modify redundant procedure

Signed-off-by: 1552980358 <1552980358@qq.com>

app/src/main/java/io/legado/app/data/entities/BookSource.kt

@@ -54,14 +54,15 @@ data class BookSource(
     fun getHeaderMap() = (HashMap<String, String>().apply {
         this[AppConst.UA_NAME] = App.INSTANCE.getPrefString("user_agent") ?: userAgent
         header?.let {
-            val header1 = when {
+            GSON.fromJsonObject<Map<String, String>>(
+                when {
                     it.startsWith("@js:", true) ->
                         evalJS(it.substring(4)).toString()
                     it.startsWith("<js>", true) ->
                         evalJS(it.substring(4, it.lastIndexOf("<"))).toString()
                     else -> it
                 }
-            GSON.fromJsonObject<Map<String, String>>(header1)?.let { map ->
+            )?.let { map ->
                 putAll(map)
             }
         }