[GEOT-6608] Fix user-agent for OSM tiles

[ianturton]

Checklist

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[aaime]

Suggested change

	headers.put("User-Agent", "Geotools Http client");
	headers.put("User-Agent", "Geotools HTTP client");

:-p

[aaime]

Wondering if SimpleHttpClient should default to the new user agent. It's not the first time I see cascading blocked because servers don't like the Java user agent.

[ianturton]

that might be an even better fix - I guess.