Fix dasht-server-http serving "400 Bad Request" on requests for "/"

This was reported in #51. It's not clear to me if this only affects
macOS users.

The problem was the `grep` query changed in this commit doesn't match queries
for "/":

    ~ $ cat urls
    /
    /?
    /?query
    /foo
    /bar
    ~ $ cat urls | grep '^/$\|^/?'
    /?
    /?query

The two patterns to match for are fairly simple, so they can instead be tested
for with simple string equality checks.

Author: Tenzer

bin/dasht-server-http

@@ -75,7 +75,7 @@ url=$(awk '
 
 # serve translated local file:// URLs over HTTP
 # (skip the / homepage and /? form submissions)
-if ! echo "$url" | grep -q '^/$\|^/?'; then
+if ! test "$url" = / -o -z "${url##/\?*}"; then
   file=${url%#*} # strip URI fragment, if any
 
   if test -f "$file"; then