fix non-unique-parameter check for post request

[tyrasd]

Description

Fixes a bug where no error is returned for non-unique request parameters in POST requests (see #12).

To reproduce the issue using javascript in a browser (note the duplicate filter parameter in both request below):

console.log((await fetch("https://api.ohsome.org/v1/elements/count?bboxes=8.67%2C49.39%2C8.71%2C49.42&filter=type%3Anode&filter=type%3Away&format=json&time=2014-01-01%2F2017-01-01%2FP1Y", { "method": "GET" })).status);
// GET request -> returns 400

console.log((await fetch("https://api.ohsome.org/v1/elements/count", {
    "headers": {
        "content-type": "application/x-www-form-urlencoded"
    },
    "method": "POST",
    "body": "bboxes=8.67%2C49.39%2C8.71%2C49.42&filter=type%3Anode&filter=type%3Away&format=json&time=2014-01-01%2F2017-01-01%2FP1Y"
})).status);
// POST request -> returns 200

This PR also simplifies the implementation for GET requests as well and add tests.

Corresponding issue

See #12

Checklist

• My code follows the code-style rules, and I have checked on the static analyses
• I have commented my code
• I have written javadoc (required for public methods)
• I have added sufficient unit and API tests
• I have made corresponding changes to the documentation
• I have updated the CHANGELOG.md
• I have adjusted the examples in the check-ohsome-api script or created an issue in the corresponding repository. More Information here.