Fix Response not appending trailing slash to collections (#130)

* Fix empty filterIsInstance in Ktor and OkHttp responses

- Change `map` to `flatMap` for properties in both Ktor and OkHttp response handling
- Ensure `filterIsInstance<ResourceType>` works correctly by flattening the properties list

* Fix tests

Author: rfc2822

src/main/kotlin/at/bitfire/dav4jvm/ktor/Response.kt

@@ -180,7 +180,7 @@ data class Response(
             // if we know this resource is a collection, make sure href has a trailing slash
             // (for clarity and resolving relative paths)
             propStat.filter { it.status.isSuccess() }
-                .map { it.properties }
+                .flatMap { it.properties }
                 .filterIsInstance<ResourceType>()
                 .firstOrNull()
                 ?.let { type ->

src/main/kotlin/at/bitfire/dav4jvm/okhttp/Response.kt

@@ -176,7 +176,7 @@ data class Response(
             // if we know this resource is a collection, make sure href has a trailing slash
             // (for clarity and resolving relative paths)
             propStat.filter { it.isSuccess() }
-                .map { it.properties }
+                .flatMap { it.properties }
                 .filterIsInstance<ResourceType>()
                 .firstOrNull()
                 ?.let { type ->

src/test/kotlin/at/bitfire/dav4jvm/ktor/DavResourceTest.kt

@@ -924,7 +924,7 @@ class DavResourceTest {
                 }
             }
         }
-        assertEquals(4, nrCalled)
+        assertEquals(5, nrCalled)
     }
 
     @Test

src/test/kotlin/at/bitfire/dav4jvm/okhttp/DavResourceTest.kt

@@ -848,7 +848,7 @@ class DavResourceTest {
                 }
             }
         }
-        assertEquals(4, nrCalled)
+        assertEquals(5, nrCalled)
 
 
         /*** SPECIAL CASES ***/