Properly syntax highlight HTTP body

Currently given an example like:

```http
GET /foo HTTP/1.1
Content-Type: application/json
User-Agent: foo

{"hello": "world"}
```

It will fail to highlight because the content type will get reset when processing the `User-Agent` header. The is particularly problematic because Go maps are randomly ordered and `http.Request` use a map for headers, meaning on some runs you could get highlighting and on others it would silently fail if you are generating the above from an `http.Request`.

This PR fixes it by resetting `isContentType` once we've read the actual content type, which prevents replacing it with later literals.

lexers/h/http.go

@@ -85,6 +85,7 @@ func (d *httpBodyContentTyper) Tokenise(options *TokeniseOptions, text string) (
 				}
 			case token.Type == Literal && isContentType:
 				{
+					isContentType = false
 					contentType = strings.TrimSpace(token.Value)
 					pos := strings.Index(contentType, ";")
 					if pos > 0 {