increase pixelfed compatibility

mastodon/src/main/java/org/joinmastodon/android/model/Account.java

@@ -65,7 +65,6 @@ public class Account extends BaseModel implements Searchable{
 	/**
 	 * An image banner that is shown above the profile and in profile cards.
 	 */
-	@RequiredField
 	public String header;
 	/**
 	 * A static version of the header. Equal to header if its value is a static image; different if header is an animated GIF.

mastodon/src/main/java/org/joinmastodon/android/model/Instance.java

@@ -148,6 +148,10 @@ public class Instance extends BaseModel{
 		return pleroma != null;
 	}
 
+	public boolean isPixelfed() {
+		return version.contains("compatible; Pixelfed");
+	}
+
 	public boolean hasFeature(Feature feature) {
 		Optional<List<String>> pleromaFeatures = Optional.ofNullable(pleroma)
 				.map(p -> p.metadata)