Fix missing custom headers for directory requests (trailing slash)

[joseluisq]

Description

This PR fixes an issue where custom headers were not applied to the response for directory requests (requests ending with the slash character).

Related Issue

Fixes #332

Motivation and Context

How Has This Been Tested?

Server

[advanced]

[[advanced.headers]]
source = "**/*.{html,htm}"
headers = { Server = "Static Web Server" }

static-web-server -w config.toml

Client requests

Request 1

curl -I "http://localhost/index.html"
# HTTP/1.1 200 OK
# content-length: 536
# content-type: text/html
# accept-ranges: bytes
# last-modified: Mon, 01 Apr 2024 22:00:03 GMT
# vary: accept-encoding
# cache-control: public, max-age=86400
# server: Static Web Server
# date: Sun, 07 Apr 2024 23:00:09 GMT

Request 2

curl -I "http://localhost/"
# HTTP/1.1 200 OK
# content-length: 536
# content-type: text/html
# accept-ranges: bytes
# last-modified: Mon, 01 Apr 2024 22:00:03 GMT
# vary: accept-encoding
# cache-control: public, max-age=86400
# server: Static Web Server
# date: Sun, 07 Apr 2024 23:00:26 GMT

Screenshots (if appropriate):

[semanticdiff-com]

Review changes with SemanticDiff.

Analyzed 7 of 8 files.

Overall, the semantic diff is 34% smaller than the GitHub diff.

	Filename	Status
✔️	src/custom_headers.rs	27.27% smaller
✔️	src/handler.rs	24.0% smaller
✔️	src/static_files.rs	52.86% smaller
✔️	tests/compression_static.rs	Analyzed
✔️	tests/dir_listing.rs	50.76% smaller
❔	tests/fixtures/toml/handler.toml	Unsupported file format
✔️	tests/handler.rs	Analyzed
✔️	tests/static_files.rs	47.49% smaller