bugfix: treat PGHOST env var as directory properly

This change makes the driver find the actual socket file and is
consistent with how libpq appends .s.PGSQL.5432 to the value of the env
var when it's a path:

    ❯ mkdir -p /tmp/foo && PGHOST=/tmp/foo psql
    psql: error: could not connect to server: No such file or directory
            Is the server running locally and accepting
            connections on Unix domain socket "/tmp/foo/.s.PGSQL.5432"?

spec/pq/conninfo_spec.cr

@@ -132,5 +132,11 @@ describe PQ::ConnInfo, ".from_conninfo_string" do
       ci.port.should eq(1)
       ci.user.should eq("D")
     end
+
+    env_var_bubble do
+      ENV["PGHOST"] = "/path"
+      ci = PQ::ConnInfo.from_conninfo_string("postgres://")
+      ci.host.should eq("/path/.s.PGSQL.5432")
+    end
   end
 end

src/pq/conninfo.cr

@@ -118,7 +118,9 @@ module PQ
     private def default_host(h)
       return h if h && !h.blank?
 
-      return ENV["PGHOST"] if ENV.has_key?("PGHOST")
+      if pghost = ENV["PGHOST"]?
+        return pghost[0] == '/' ? "#{pghost}/.s.PGSQL.5432" : pghost
+      end
 
       SOCKET_SEARCH.each do |s|
         return s if File.exists?(s)