[Generator] use absolute xcasset file path in copy resource script

Wrong `xcassets` path in generated `Pods-resources.sh` file. This bug
can be reproduced by adding [1PasswordExtension
spec](https://github.com/CocoaPods/Specs/blob/master/Specs/1PasswordExtension/1.1.2/1PasswordExtension.podspec.json)
to demo app.

lib/cocoapods/generator/copy_resources_script.rb

@@ -128,7 +128,7 @@ def script
       xcrun mapc "${PODS_ROOT}/$1" "${CONFIGURATION_BUILD_DIR}/${UNLOCALIZED_RESOURCES_FOLDER_PATH}/`basename "$1" .xcmappingmodel`.cdm"
       ;;
     *.xcassets)
-      XCASSET_FILES="$XCASSET_FILES '$1'"
+      XCASSET_FILES="$XCASSET_FILES '${PODS_ROOT}/$1'"
       ;;
     /*)
       echo "$1"