Single value header gets parsed as Vec

Nobody has claimed this yet.

I have encountered an interesting bug - trying to match a header value which contains a comma `,` results in incoming header value being parsed as Vec

What doesn't work, but I think it should
```
let mock_server = MockServer::start().await;

Mock::given(header("if-modified-since", "Sat, 02 Apr 2005 20:37:00 GMT")) // This is valid RFC2822
.respond_with(ResponseTemplate::new(304))
.expect(1)
.mount(&mock_server)
.await;

// This will return 404
let _result = reqwest::Client::new()
.get(mock_server.uri())
.header("if-modified-since", "Sat, 02 Apr 2005 20:37:00 GMT")
.send()
.await
.unwrap()
.error_for_status()
.unwrap();
```

Workaround I found:
```
let mock_server = MockServer::start().await;

Mock::given(headers(
"if-modified-since",
vec!["Sat", "02 Apr 2005 20:37:00 GMT"],
))
.respond_with(ResponseTemplate::new(304))
.expect(1)
.mount(&mock_server)
.await;

// Returns 304
let _result = reqwest::Client::new()
.get(mock_server.uri())
.header("if-modified-since", "Sat, 02 Apr 2005 20:37:00 GMT")
.send()
.await
.unwrap()
.error_for_status()
.unwrap();
}
```

I have prepared a repository with a test that reproduces the issue: https://github.com/muttleyxd/wiremock-header-matcher-bug/blob/master/src/main.rs

Start by running the reproduction in src/main.rs from the linked repository and compare the single-value header matcher with the headers workaround. Trace the header matcher entry point used by Mock::given(header(...)). Done means a header value containing a comma matches the request as one value, with regression coverage for the reported case.

Written by the indexing model from the issue text.