#3957 added string file parameter to getStrings request for iOS.

Pull request for Issue #3957. 
I added a parameter stringFile to the getAppStrings request for iOS. Tested it with my app and its working.

One strange behavior occurs when passing a file name that does not exists I get the content of the Localizable.strings. Shouldn't be the response value empty? Here the log output.

info: --> POST /wd/hub/session/eb260020-f235-400d-95f6-bcef06e92c4a/appium/app/strings {"language":"en","stringFile":"xxx"}
info: [debug] No strings file 'xxx' for language 'en', getting default strings
info: [debug] Could not parse plist file (as binary) at /Users/me/Library/Developer/Xcode/DerivedData/MyApp-dspsyeiacfmescfzmsleijgpgcwk/Build/Products/Debug-iphonesimulator/Idealo.app/en.lproj/xxx
info: Will try to parse the plist file as XML
`parseFileSync()` is deprecated. Use `parseStringSync()` instead.
info: [debug] Could not parse plist file (as XML) at /Users/me/Library/Developer/Xcode/DerivedData/MyApp-dspsyeiacfmescfzmsleijgpgcwk/Build/Products/Debug-iphonesimulator/Idealo.app/en.lproj/xxx
warn: Could not parse app xxx assuming it doesn't exist
info: [debug] Responding to client with success: {"status":0,"value":{"PUSH_ALERT":"Congratulations! Your product has reached its target price."},"sessionId":"eb260020-f235-400d-95f6-bcef06e92c4a"}
info: <-- POST /wd/hub/session/eb260020-f235-400d-95f6-bcef06e92c4a/appium/app/strings 200 4.955 ms - 155 {"status":0,"value":{"PUSH_ALERT":"Congratulations! Your product has reached its target price."},"sessionId":"eb260020-f235-400d-95f6-bcef06e92c4a"}
info: --> DELETE /wd/hub/session/eb260020-f235-400d-95f6-bcef06e92c4a {}
let's pass `null` instead of `undefined` as `undefined` could conceivably be overridden