set JSON request body encoding to UTF-8 (it was ISO_8859_1 by default)

[D3DXInitPerDX]

per JSON specification JSON strings SHALL be encoded in UTF-8 (or other UTF encodings).
the constructor StringEntity(String) uses ISO_8859_1 encoding by default. This mistake leads to the error from szurubooru api: "Could not decode the request body. The JSON was incorrect or was not encoded as UTF-8" if json contains unusual symbols (like ×).

(the latest release does not implement any API that could be broken by this bug, I encountered it with my extended SzurubooruAPI that creates post with pre-defined tags and source:

public SzurubooruPost createPost(FileToken content, String safety, List<String> tags, String source) throws IOException, SzurubooruHTTPException, SzurubooruResourceNotSynchronizedException {
            if (!SzurubooruPost.isValidSafety(safety)) {
                throw new IllegalArgumentException("Safety must be of value safe, sketchy or unsafe");
            } else {

                List<Map<String, Object>> tagsMap = tags.stream()
                        .map(this::truncateTag)
                        .map(x -> Map.of("names", (Object) List.of(x)))
                        .toList();
                SzurubooruPost post = new SzurubooruPost(this, new HashMap<>());
                post.setNewJson(Map.of("tags", tagsMap, "safety", safety, "contentToken", content.getToken(),
                        "source", source));
                post.push();
                return post;
            }
        }

        // szurubooru does not accept tags longer than 128 in length
        private String truncateTag(String tag) {
            return tag.length() > 128 ? tag.substring(0, 128) : tag;
        }

So, if any tag or the source contains "×", the mentioned error will occur. This commit fixes that.

[D3DXInitPerDX]

@G1org1owo please look at this merge request.

[G1org1owo]

Thanks for the contribution! I didn't think someone would actually use this lib so you've made me quite happy :)

If there is anything else that needs fixing/improving I'm open to ideas!

[D3DXInitPerDX]

Glad I could help. Right now i'm using only a small fraction of the API (my self-hosted szuru is pretty simple structurally), so no any good ideas regarding the functionality. Maybe later I'll think of something.