Make sure Western digits are used where Western digits are required

src/main/kotlin/at/bitfire/dav4jvm/BasicDigestAuthHandler.kt

@@ -172,7 +172,7 @@ class BasicDigestAuthHandler(
             params.add("cnonce=${quotedString(clientNonce)}")
 
             val nc = nonceCount.getAndIncrement()
-            val ncValue = String.format("%08x", nc)
+            val ncValue = String.format(Locale.ROOT, "%08x", nc)
             params.add("nc=$ncValue")
 
             val a1: String? = when (algorithm) {

src/main/kotlin/at/bitfire/dav4jvm/DavCalendar.kt

@@ -44,7 +44,7 @@ class DavCalendar @JvmOverloads constructor(
         const val TIME_RANGE_START = "start"
         const val TIME_RANGE_END = "end"
 
-        private val timeFormatUTC = SimpleDateFormat("yyyyMMdd'T'HHmmss'Z'", Locale.US)
+        private val timeFormatUTC = SimpleDateFormat("yyyyMMdd'T'HHmmss'Z'", Locale.ROOT)
         init {
             timeFormatUTC.timeZone = TimeZone.getTimeZone("Etc/UTC")
         }

src/main/kotlin/at/bitfire/dav4jvm/HttpUtils.kt

@@ -17,7 +17,7 @@ import java.util.*
 object HttpUtils {
 
     const val httpDateFormatStr = "EEE, dd MMM yyyy HH:mm:ss 'GMT'"
-    val httpDateFormat = SimpleDateFormat(httpDateFormatStr)
+    val httpDateFormat = SimpleDateFormat(httpDateFormatStr, Locale.ROOT)
 
     /**
      * Gets the resource name (the last segment of the path) from an URL.