Update `nvm_ensure_version_prefix` to support `iojs`-prefixed version strings.

nvm.sh

@@ -211,7 +211,13 @@ nvm_normalize_version() {
 }
 
 nvm_ensure_version_prefix() {
-  echo "$1" | command sed -e 's/^\([0-9]\)/v\1/g'
+  local NVM_VERSION
+  NVM_VERSION="$(nvm_strip_iojs_prefix "$1" | command sed -e 's/^\([0-9]\)/v\1/g')"
+  if nvm_is_iojs_version "$1"; then
+    echo "$(nvm_add_iojs_prefix "$NVM_VERSION")"
+  else
+    echo "$NVM_VERSION"
+  fi
 }
 
 nvm_format_version() {

test/fast/Unit tests/nvm_ensure_version_prefix

@@ -7,3 +7,6 @@ die () { echo $@ ; exit 1; }
 [ "_$(nvm_ensure_version_prefix 1)" = "_v1" ] || die '"nvm_ensure_version_prefix 1" did not return "v1"'
 [ "_$(nvm_ensure_version_prefix v1)" = "_v1" ] || die '"nvm_ensure_version_prefix v1" did not return "v1"'
 [ "_$(nvm_ensure_version_prefix foo)" = "_foo" ] || die '"nvm_ensure_version_prefix foo" did not return "foo"'
+
+[ "_$(nvm_ensure_version_prefix iojs-1)" = "_iojs-v1" ] || die '"nvm_ensure_version_prefix iojs-1" did not return "iojs-v1"'
+[ "_$(nvm_ensure_version_prefix iojs-v1)" = "_iojs-v1" ] || die '"nvm_ensure_version_prefix iojs-v1" did not return "iojs-v1"'