[Fix] nvm_strip_path: Preserve leading/trailing colons

Path lists in environmental variables often give special meaning to
empty entries (e.g. in PATH or MANPATH). These are represented by
leading or trailing colons, or by doubled colons in the middle of the
list.

Adjust the awk invocation to correctly deal with trailing colons by
printing the separator before every field except the first, and then
printing the final separator that is read from the input - this will
either be a colon or the null string. This preserves leading and
trailing colons in all cases while not adding extra colons in the wrong
place.

Add test to confirm the correct behaviour.

Fixes nvm-sh#3144

nvm.sh

@@ -823,7 +823,8 @@ nvm_strip_path() {
     path = substr($0, length(NVM_DIR) + 1)
     if (path ~ "^(/versions/[^/]*)?/[^/]*'"${2-}"'.*$") { next }
   }
-  { print }' | command paste -s -d: -
+  # The final RT will contain a colon if the input has a trailing colon, or a null string otherwise
+  { printf "%s%s", sep, $0; sep=RS } END { printf "%s", RT }'
 }
 
 nvm_change_path() {

test/fast/Unit tests/nvm_strip_path

@@ -16,3 +16,9 @@ TEST_PATH="$NVM_DIR/v0.10.5/bin:/usr/bin:$NVM_DIR/v0.11.5/bin:$NVM_DIR/v0.9.5/bi
 STRIPPED_PATH=`nvm_strip_path "$TEST_PATH" "/bin"`
 
 [ "$STRIPPED_PATH" = "/usr/bin:/usr/local/bin" ] || die "Not correctly stripped: $STRIPPED_PATH "
+
+TEST_PATH=":/a/b/bin::/c/d/bin:"
+
+STRIPPED_PATH=`nvm_strip_path "$TEST_PATH" "/bin"`
+
+[ "$STRIPPED_PATH" = "$TEST_PATH" ] || die "Stripping does not preserve colons: $STRIPPED_PATH "